diff --git a/.yarn/sdks/typescript/package.json b/.yarn/sdks/typescript/package.json
index 19a0b6f1886..27ab4b140b1 100644
--- a/.yarn/sdks/typescript/package.json
+++ b/.yarn/sdks/typescript/package.json
@@ -1,6 +1,6 @@
 {
   "name": "typescript",
-  "version": "5.9.3-sdk",
+  "version": "6.0.2-sdk",
   "main": "./lib/typescript.js",
   "type": "commonjs",
   "bin": {
diff --git a/app/javascript/@types/assets.d.ts b/app/javascript/@types/assets.d.ts
index cdb2b1a9a23..8b6b6ba3bf7 100644
--- a/app/javascript/@types/assets.d.ts
+++ b/app/javascript/@types/assets.d.ts
@@ -2,3 +2,5 @@ declare module '*.svg' {
   const content: string;
   export default content;
 }
+
+declare module 'vite/modulepreload-polyfill';
diff --git a/app/javascript/Authentication/EditUser.tsx b/app/javascript/Authentication/EditUser.tsx
index 1659a2b856a..7fe17096acf 100644
--- a/app/javascript/Authentication/EditUser.tsx
+++ b/app/javascript/Authentication/EditUser.tsx
@@ -103,7 +103,11 @@ function EditUserForm() {
         <div className="card-header">{t('authentication.editUser.accountDataHeader')}</div>
 
         <div className="card-body">
-          <UserFormFields formState={formState} setFormState={setFormState} showNameWarning />
+          <UserFormFields
+            formState={formState}
+            setFormState={setFormState as React.Dispatch<React.SetStateAction<UserFormState>>}
+            showNameWarning
+          />
           <div className="mb-3">
             <label className="form-label" htmlFor={passwordFieldId}>
               {t('authentication.editUser.passwordLabel')}
diff --git a/app/javascript/Authentication/SignInForm.tsx b/app/javascript/Authentication/SignInForm.tsx
index 9fa8d1d55eb..51e3086b2db 100644
--- a/app/javascript/Authentication/SignInForm.tsx
+++ b/app/javascript/Authentication/SignInForm.tsx
@@ -72,8 +72,8 @@ function SignInForm(): React.JSX.Element {
         autoDismissAfter: 1000 * 60,
       });
     } catch (e) {
-      if (!e.message.match(/invalid email or password/i)) {
-        errorReporting().error(e);
+      if (!(e instanceof Error && e.message.match(/invalid email or password/i))) {
+        errorReporting().error(e as string | Error);
       }
 
       // we're doing suppressError below specifically so that we can not capture invalid email
diff --git a/app/javascript/AuthenticityTokensContext.tsx b/app/javascript/AuthenticityTokensContext.tsx
index 3bcdfa236e0..fc90391990f 100644
--- a/app/javascript/AuthenticityTokensContext.tsx
+++ b/app/javascript/AuthenticityTokensContext.tsx
@@ -33,7 +33,9 @@ export default class AuthenticityTokensManager {
 
     if (
       Object.keys(newTokens).length === Object.keys(prevTokens ?? {}).length &&
-      Object.keys(prevTokens).every((key: keyof typeof prevTokens) => newTokens[key] !== prevTokens[key])
+      Object.keys(prevTokens).every(
+        (key) => newTokens[key as keyof typeof prevTokens] !== prevTokens[key as keyof typeof prevTokens],
+      )
     ) {
       return;
     }
diff --git a/app/javascript/BuiltInFormControls/GraphQLAsyncSelect.tsx b/app/javascript/BuiltInFormControls/GraphQLAsyncSelect.tsx
index f130acd1085..3d496c9f02f 100644
--- a/app/javascript/BuiltInFormControls/GraphQLAsyncSelect.tsx
+++ b/app/javascript/BuiltInFormControls/GraphQLAsyncSelect.tsx
@@ -5,21 +5,30 @@ import { useApolloClient } from '@apollo/client/react';
 
 // This will be a lot more useful once https://github.com/microsoft/TypeScript/issues/36981
 // is fixed
-export type GraphQLAsyncSelectProps<QueryType extends TypedDocumentNode, OptionType, IsMulti extends boolean> = Omit<
-  AsyncProps<OptionType, IsMulti, GroupBase<OptionType>>,
-  'loadOptions'
-> & {
+// TypedDocumentNode<any, any> is needed because TypeScript 6 now correctly checks variance
+// of phantom type parameters in TypedDocumentNode, and only `any` bypasses this constraint.
+/* eslint-disable @typescript-eslint/no-explicit-any */
+export type GraphQLAsyncSelectProps<
+  QueryType extends TypedDocumentNode<any, any>,
+  OptionType,
+  IsMulti extends boolean,
+> = Omit<AsyncProps<OptionType, IsMulti, GroupBase<OptionType>>, 'loadOptions'> & {
   query: QueryType;
   getVariables: (inputValue: string) => VariablesOf<QueryType>;
   getOptions: (results: ResultOf<QueryType>) => OptionType[];
 };
 
-function GraphQLAsyncSelect<QueryType extends TypedDocumentNode, OptionType, IsMulti extends boolean = false>({
+function GraphQLAsyncSelect<
+  QueryType extends TypedDocumentNode<any, any>,
+  OptionType,
+  IsMulti extends boolean = false,
+>({
   query,
   getOptions,
   getVariables,
   ...otherProps
 }: GraphQLAsyncSelectProps<QueryType, OptionType, IsMulti>): React.JSX.Element {
+  /* eslint-enable @typescript-eslint/no-explicit-any */
   const client = useApolloClient();
   const loadOptions = async (inputValue: string) => {
     try {
diff --git a/app/javascript/BuiltInFormControls/ScheduledValueTimespanRow.tsx b/app/javascript/BuiltInFormControls/ScheduledValueTimespanRow.tsx
index effefe5e34e..df4532613e8 100644
--- a/app/javascript/BuiltInFormControls/ScheduledValueTimespanRow.tsx
+++ b/app/javascript/BuiltInFormControls/ScheduledValueTimespanRow.tsx
@@ -41,7 +41,7 @@ function ScheduledValueTimespanRow<ValueType>({
   );
 
   const valueChanged = useCallback(
-    (value: ValueType) => valueDidChange(rowIdentifier, value),
+    (value: ValueType | undefined) => valueDidChange(rowIdentifier, value),
     [valueDidChange, rowIdentifier],
   );
 
diff --git a/app/javascript/BuiltInFormControls/UserConProfileSelect.tsx b/app/javascript/BuiltInFormControls/UserConProfileSelect.tsx
index d191a9c90d0..4e632b11d39 100644
--- a/app/javascript/BuiltInFormControls/UserConProfileSelect.tsx
+++ b/app/javascript/BuiltInFormControls/UserConProfileSelect.tsx
@@ -14,12 +14,19 @@ type DO<QueryType extends DefaultUserConProfilesQueryData> = NonNullable<
   QueryType['convention']
 >['user_con_profiles_paginated']['entries'][0];
 
-export type UserConProfileSelectProps<QueryType extends TypedDocumentNode, OptionType, IsMulti extends boolean> = Omit<
+/* eslint-disable @typescript-eslint/no-explicit-any */
+// TypedDocumentNode<any, any> needed because TypeScript 6 variance checks require it
+export type UserConProfileSelectProps<
+  QueryType extends TypedDocumentNode<any, any>,
+  OptionType,
+  IsMulti extends boolean,
+> = Omit<
   GraphQLAsyncSelectProps<QueryType, OptionType, IsMulti>,
   'isClearable' | 'getOptions' | 'getVariables' | 'getOptionValue' | 'formatOptionLabel' | 'query'
 > & {
   userConProfilesQuery?: DocumentNode;
 };
+/* eslint-enable @typescript-eslint/no-explicit-any */
 
 function UserConProfileSelect<
   IsMulti extends boolean = false,
diff --git a/app/javascript/BuiltInFormControls/UserSelect.tsx b/app/javascript/BuiltInFormControls/UserSelect.tsx
index 93eb677390c..17ea1966037 100644
--- a/app/javascript/BuiltInFormControls/UserSelect.tsx
+++ b/app/javascript/BuiltInFormControls/UserSelect.tsx
@@ -11,7 +11,6 @@ type UserNameLabelProps<OptionType, IsMulti extends boolean> = MultiValueGeneric
     name?: string;
   };
   children: ReactNode;
-  [x: string]: unknown;
 };
 
 function UserNameLabel<OptionType, IsMulti extends boolean>({
diff --git a/app/javascript/BuiltInForms/FileUploadForm.tsx b/app/javascript/BuiltInForms/FileUploadForm.tsx
index 6218d2bf3b8..a13907b4e4d 100644
--- a/app/javascript/BuiltInForms/FileUploadForm.tsx
+++ b/app/javascript/BuiltInForms/FileUploadForm.tsx
@@ -9,7 +9,11 @@ import RailsDirectUploadsContext from '../RailsDirectUploadsContext';
 import classNames from 'classnames';
 import { AuthenticityTokensContext } from '../AuthenticityTokensContext';
 
-function uploadFile(file: File, directUploadURL: string, onProgress?: (event: ProgressEvent<XMLHttpRequest>) => void) {
+function uploadFile(
+  file: File,
+  directUploadURL: string,
+  onProgress?: (event: ProgressEvent<XMLHttpRequestEventTarget>) => void,
+) {
   return new Promise<Blob | undefined>((resolve, reject) => {
     const delegate: DirectUploadDelegate = {
       directUploadWillStoreFileWithXHR: (xhr) => {
@@ -44,7 +48,7 @@ function FileUploadForm({ onUpload }: FileUploadFormProps): React.JSX.Element {
   const manager = useContext(AuthenticityTokensContext);
   const directUploadsAuthenticityToken = manager.tokens?.railsDirectUploads;
 
-  const onProgress = useCallback((event: ProgressEvent<XMLHttpRequest>) => {
+  const onProgress = useCallback((event: ProgressEvent<XMLHttpRequestEventTarget>) => {
     setProgressIndeterminate(!event.lengthComputable);
     if (event.lengthComputable) {
       setProgressPercent((event.loaded / event.total) * 100);
diff --git a/app/javascript/BuiltInForms/RunFormFields.tsx b/app/javascript/BuiltInForms/RunFormFields.tsx
index 7f39aca86c5..5565170c7de 100644
--- a/app/javascript/BuiltInForms/RunFormFields.tsx
+++ b/app/javascript/BuiltInForms/RunFormFields.tsx
@@ -130,7 +130,7 @@ function RunFormFields<RunType extends RunForRunFormFields>({
             rooms={convention.rooms ?? []}
             isMulti
             value={run.rooms}
-            onChange={(rooms: RoomForSelect[]) => onChange((prevRun) => ({ ...prevRun, rooms }))}
+            onChange={(rooms) => onChange((prevRun) => ({ ...prevRun, rooms: [...rooms] }))}
           />
         )}
       </FormGroupWithLabel>
diff --git a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/CmsContentGroupFormFields.tsx b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/CmsContentGroupFormFields.tsx
index 74b1cd0b92b..6aa52617f7a 100644
--- a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/CmsContentGroupFormFields.tsx
+++ b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/CmsContentGroupFormFields.tsx
@@ -3,7 +3,7 @@ import * as React from 'react';
 import uniqWith from 'lodash/uniqWith';
 import { BootstrapFormInput, FormGroupWithLabel, notEmpty } from '@neinteractiveliterature/litform';
 
-import CmsContentSelect, { CmsContentOption } from './CmsContentSelect';
+import CmsContentSelect from './CmsContentSelect';
 import PermissionsTableInput from '../../Permissions/PermissionsTableInput';
 import SelectWithLabel from '../../BuiltInFormControls/SelectWithLabel';
 import { permissionEquals, getPermissionNamesForModelType } from '../../Permissions/PermissionUtils';
@@ -84,9 +84,7 @@ function CmsContentGroupFormFields({
             name="contents"
             value={contentGroup.contents}
             inputId={id}
-            onChange={(contents: CmsContentOption[]) =>
-              setContentGroup && setContentGroup({ ...contentGroup, contents })
-            }
+            onChange={(contents) => setContentGroup && setContentGroup({ ...contentGroup, contents: [...contents] })}
             isDisabled={disabled || readOnly}
           />
         )}
@@ -111,7 +109,7 @@ function CmsContentGroupFormFields({
               }}
               readOnly={readOnly}
               changeSet={permissionsChangeSet}
-              add={addPermission}
+              add={addPermission as UsePermissionsChangeSetOptions['add']}
               remove={removePermission}
             />
           )}
@@ -122,7 +120,7 @@ function CmsContentGroupFormFields({
               options={convention?.staff_positions ?? []}
               getOptionValue={(staffPosition) => staffPosition.id.toString()}
               getOptionLabel={(staffPosition) => staffPosition.name}
-              onChange={addRole}
+              onChange={(role) => role && addRole(role)}
             />
           )}
         </div>
diff --git a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/EditCmsContentGroup.tsx b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/EditCmsContentGroup.tsx
index 2ba08729dc1..2397be3a192 100644
--- a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/EditCmsContentGroup.tsx
+++ b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/EditCmsContentGroup.tsx
@@ -13,7 +13,7 @@ import { ErrorDisplay, notEmpty } from '@neinteractiveliterature/litform';
 
 import { buildPermissionInput } from '../../Permissions/PermissionUtils';
 import { useChangeSet } from '../../ChangeSet';
-import CmsContentGroupFormFields from './CmsContentGroupFormFields';
+import CmsContentGroupFormFields, { CmsContentGroupFormFieldsProps } from './CmsContentGroupFormFields';
 import { CmsContentGroupsAdminQueryData } from './queries.generated';
 import { CmsContentTypeIndicator } from '../../graphqlTypes.generated';
 import { singleCmsContentGroupAdminLoader, SingleCmsContentGroupAdminLoaderResult } from './loaders';
@@ -89,7 +89,7 @@ function EditCmsContentGroupForm() {
         disabled={submitInProgress}
         convention={convention}
         permissionsChangeSet={permissionsChangeSet}
-        addPermission={addPermission}
+        addPermission={addPermission as unknown as CmsContentGroupFormFieldsProps['addPermission']}
         removePermission={removePermission}
       />
 
diff --git a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/NewCmsContentGroup.tsx b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/NewCmsContentGroup.tsx
index 9578dec9d9d..84d59e0f7ea 100644
--- a/app/javascript/CmsAdmin/CmsContentGroupsAdmin/NewCmsContentGroup.tsx
+++ b/app/javascript/CmsAdmin/CmsContentGroupsAdmin/NewCmsContentGroup.tsx
@@ -6,7 +6,7 @@ import { ErrorDisplay } from '@neinteractiveliterature/litform';
 import { buildPermissionInput } from '../../Permissions/PermissionUtils';
 import { useChangeSet } from '../../ChangeSet';
 import usePageTitle from '../../usePageTitle';
-import CmsContentGroupFormFields from './CmsContentGroupFormFields';
+import CmsContentGroupFormFields, { CmsContentGroupFormFieldsProps } from './CmsContentGroupFormFields';
 import { CmsContentGroupsAdminQueryData } from './queries.generated';
 import { CmsContentTypeIndicator, CreateCmsContentGroupInput } from '../../graphqlTypes.generated';
 import { useCmsContentGroupsAdminLoader } from './loaders';
@@ -79,7 +79,7 @@ function NewCmsContentGroup(): React.JSX.Element {
         disabled={createInProgress}
         convention={data.convention}
         permissionsChangeSet={permissionsChangeSet}
-        addPermission={addPermission}
+        addPermission={addPermission as unknown as CmsContentGroupFormFieldsProps['addPermission']}
         removePermission={removePermission}
       />
 
diff --git a/app/javascript/CmsAdmin/NavigationItemsAdmin/Client.ts b/app/javascript/CmsAdmin/NavigationItemsAdmin/Client.ts
index 31b7a91643d..c4d5c051dea 100644
--- a/app/javascript/CmsAdmin/NavigationItemsAdmin/Client.ts
+++ b/app/javascript/CmsAdmin/NavigationItemsAdmin/Client.ts
@@ -81,7 +81,7 @@ class Client implements CadmusNavbarAdminClient {
       });
       return data!.cmsParent.cmsNavigationItems.map(graphqlNavigationItemToCadmusNavbarAdminObject);
     } catch (error) {
-      this.onError(error);
+      this.onError(error as Error);
       throw error;
     } finally {
       this.requestsInProgress.loadingNavigationItems = false;
@@ -105,7 +105,7 @@ class Client implements CadmusNavbarAdminClient {
         name: page.name ?? 'Untitled page',
       }));
     } catch (error) {
-      this.onError(error);
+      this.onError(error as Error);
       throw error;
     } finally {
       this.requestsInProgress.loadingPages = false;
@@ -150,7 +150,7 @@ class Client implements CadmusNavbarAdminClient {
       await this.apolloClient.resetStore();
       return graphqlNavigationItemToCadmusNavbarAdminObject(mutationResponse.cms_navigation_item);
     } catch (error) {
-      this.onError(error);
+      this.onError(error as Error);
       throw error;
     } finally {
       this.requestsInProgress.savingNavigationItem = false;
@@ -168,7 +168,7 @@ class Client implements CadmusNavbarAdminClient {
       });
       await this.apolloClient.resetStore();
     } catch (error) {
-      this.onError(error);
+      this.onError(error as Error);
       throw error;
     } finally {
       this.requestsInProgress.deletingNavigationItem = false;
@@ -193,7 +193,7 @@ class Client implements CadmusNavbarAdminClient {
 
       await this.apolloClient.resetStore();
     } catch (error) {
-      this.onError(error);
+      this.onError(error as Error);
       throw error;
     } finally {
       this.requestsInProgress.sortingNavigationItems = false;
diff --git a/app/javascript/ConventionAdmin/ConventionFormGeneralSection.tsx b/app/javascript/ConventionAdmin/ConventionFormGeneralSection.tsx
index d8f36bdf228..39a23b187c5 100644
--- a/app/javascript/ConventionAdmin/ConventionFormGeneralSection.tsx
+++ b/app/javascript/ConventionAdmin/ConventionFormGeneralSection.tsx
@@ -124,7 +124,11 @@ function ConventionFormGeneralSection({
         disabled={disabled}
       />
 
-      <ConventionLanguageInput value={convention.language} onChange={setLanguage} disabled={disabled} />
+      <ConventionLanguageInput
+        value={convention.language}
+        onChange={setLanguage as React.Dispatch<React.SetStateAction<string | null | undefined>>}
+        disabled={disabled}
+      />
 
       <BootstrapFormInput
         name="domain"
diff --git a/app/javascript/ConventionAdmin/ConventionFormWebsiteSection.tsx b/app/javascript/ConventionAdmin/ConventionFormWebsiteSection.tsx
index 8c635f63831..08ea59a5b19 100644
--- a/app/javascript/ConventionAdmin/ConventionFormWebsiteSection.tsx
+++ b/app/javascript/ConventionAdmin/ConventionFormWebsiteSection.tsx
@@ -59,7 +59,7 @@ function ConventionFormWebsiteSection({
         getOptionValue={(option) => option?.id.toString() ?? ''}
         getOptionLabel={(option) => option?.name ?? ''}
         options={cmsLayouts}
-        onChange={(newValue: typeof convention.defaultLayout) => setDefaultLayout(newValue)}
+        onChange={(newValue) => newValue && setDefaultLayout(newValue)}
         styles={selectStyles}
         isDisabled={disabled}
       />
@@ -71,7 +71,7 @@ function ConventionFormWebsiteSection({
         getOptionValue={(option) => option?.id.toString() ?? ''}
         getOptionLabel={(option) => option?.name ?? ''}
         options={pages}
-        onChange={(newValue: typeof convention.rootPage) => setRootPage(newValue)}
+        onChange={(newValue) => newValue && setRootPage(newValue)}
         styles={selectStyles}
         isDisabled={disabled}
       />
diff --git a/app/javascript/DevModeGraphiql.tsx b/app/javascript/DevModeGraphiql.tsx
index ae8ad2ddd96..ca69e07f097 100644
--- a/app/javascript/DevModeGraphiql.tsx
+++ b/app/javascript/DevModeGraphiql.tsx
@@ -1,7 +1,7 @@
 import React, { useCallback, useContext, useEffect } from 'react';
 import { GraphiQL } from 'graphiql';
 import { parse } from 'graphql';
-import { Fetcher } from '@graphiql/toolkit';
+import { Fetcher, FetcherParams } from '@graphiql/toolkit';
 
 import { ApolloLink, execute } from '@apollo/client';
 import { useIntercodeApolloLink } from './useIntercodeApolloClient';
@@ -27,15 +27,15 @@ function DevModeGraphiql({ authenticityTokens }: DevModeGraphiqlProps): React.JS
   const link = useIntercodeApolloLink(new URL('/graphql', window.location.href), manager);
   const client = useApolloClient();
 
-  const fetcher: Fetcher = useCallback(
-    (operation) => {
+  const fetcher = useCallback(
+    (operation: FetcherParams) => {
       const operationAsGraphQLRequest = operation as unknown as ApolloLink.Request;
 
       operationAsGraphQLRequest.query = parse(operation.query);
       return execute(link, operationAsGraphQLRequest, { client });
     },
     [link, client],
-  );
+  ) as unknown as Fetcher;
 
   return <GraphiQL fetcher={fetcher} editorTheme="intercode" />;
 }
diff --git a/app/javascript/EventAdmin/EditRunModal.tsx b/app/javascript/EventAdmin/EditRunModal.tsx
index 8dc44d90e11..e3728bbd50f 100644
--- a/app/javascript/EventAdmin/EditRunModal.tsx
+++ b/app/javascript/EventAdmin/EditRunModal.tsx
@@ -58,7 +58,12 @@ function EditRunModal({
         </div>
         {run && (
           <div className="modal-body">
-            <RunFormFields convention={convention} run={run} event={event} onChange={editingRunChanged} />
+            <RunFormFields
+              convention={convention}
+              run={run as RunFieldsFragment}
+              event={event}
+              onChange={editingRunChanged as React.Dispatch<React.SetStateAction<RunFieldsFragment>>}
+            />
           </div>
         )}
         <div className="modal-footer">
diff --git a/app/javascript/EventAdmin/NewEvent.tsx b/app/javascript/EventAdmin/NewEvent.tsx
index 1db8d75cf38..377173b355e 100644
--- a/app/javascript/EventAdmin/NewEvent.tsx
+++ b/app/javascript/EventAdmin/NewEvent.tsx
@@ -116,7 +116,7 @@ function NewEvent() {
         encType: 'application/json',
       });
     } catch (error) {
-      setCreateError(error);
+      setCreateError(error instanceof Error ? error : undefined);
       throw error;
     }
   };
diff --git a/app/javascript/EventAdmin/useEventForm.tsx b/app/javascript/EventAdmin/useEventForm.tsx
index 3a49e7af454..22eaca16a5e 100644
--- a/app/javascript/EventAdmin/useEventForm.tsx
+++ b/app/javascript/EventAdmin/useEventForm.tsx
@@ -121,8 +121,8 @@ export default function useEventForm<
   const { formRef, validate, itemInteractionTrackingProps } = useValidatableForm();
 
   const formResponseValuesChanged = useCallback(
-    (newResponseValues: EventType['form_response_attrs']) => {
-      const processedResponseValues = Object.entries(newResponseValues).reduce(
+    (newResponseValues: unknown) => {
+      const processedResponseValues = Object.entries(newResponseValues as EventType['form_response_attrs']).reduce(
         (processed, [key, value]) => ({
           ...processed,
           ...processFormResponseValue(key, value),
diff --git a/app/javascript/EventProposals/CreateEventProposalModal.tsx b/app/javascript/EventProposals/CreateEventProposalModal.tsx
index dcb97e08bc2..5cdbbe43691 100644
--- a/app/javascript/EventProposals/CreateEventProposalModal.tsx
+++ b/app/javascript/EventProposals/CreateEventProposalModal.tsx
@@ -83,7 +83,7 @@ function CreateEventProposalModal({
           value={department || (eventCategory && eventCategory.department == null ? eventCategory : null)}
           getOptionValue={(option) => `${option.__typename}:${option.id}`}
           getOptionLabel={(option) => option.name}
-          onChange={(entity: (typeof topLevelEntities)[0]) => {
+          onChange={(entity) => {
             if (!entity) {
               setDepartment(undefined);
               setEventCategory(undefined);
@@ -111,9 +111,7 @@ function CreateEventProposalModal({
               value={eventCategory}
               getOptionValue={(option) => option.id.toString()}
               getOptionLabel={(option) => option.name}
-              onChange={(category: (typeof departmentEventCategories)[0]) => {
-                setEventCategory(category);
-              }}
+              onChange={(category) => setEventCategory(category ?? undefined)}
             />
           </>
         )}
@@ -142,9 +140,7 @@ function CreateEventProposalModal({
           value={cloneEventProposal}
           getOptionValue={(option) => option.id.toString()}
           getOptionLabel={(option) => `${option.title} (${option.event_category.name}, ${option.convention.name})`}
-          onChange={(proposal: (typeof userEventProposals)[0]) => {
-            setCloneEventProposal(proposal);
-          }}
+          onChange={(proposal) => setCloneEventProposal(proposal ?? undefined)}
         />
 
         {cloneEventProposal &&
diff --git a/app/javascript/EventProposals/EventProposalForm.tsx b/app/javascript/EventProposals/EventProposalForm.tsx
index 5fca4952a74..de9ea622403 100644
--- a/app/javascript/EventProposals/EventProposalForm.tsx
+++ b/app/javascript/EventProposals/EventProposalForm.tsx
@@ -51,18 +51,18 @@ function EventProposalFormInner({
     return data?.attachImageToEventProposal.attachment;
   });
 
-  const responseValuesChanged = useCallback(
-    (newResponseValues: (typeof initialEventProposal)['form_response_attrs']) => {
-      setEventProposal((prevEventProposal) => ({
-        ...prevEventProposal,
-        form_response_attrs: {
-          ...prevEventProposal.form_response_attrs,
-          ...newResponseValues,
-        },
-      }));
-    },
-    [],
-  );
+  const responseValuesChanged = useCallback((newResponseValues: Record<string, unknown>) => {
+    setEventProposal(
+      (prevEventProposal) =>
+        ({
+          ...prevEventProposal,
+          form_response_attrs: {
+            ...prevEventProposal.form_response_attrs,
+            ...newResponseValues,
+          },
+        }) as typeof prevEventProposal,
+    );
+  }, []);
 
   const commitResponse = useCallback(async (proposal: typeof eventProposal) => {
     try {
@@ -81,7 +81,7 @@ function EventProposalFormInner({
       setUpdatePromise(promise);
       await promise;
     } catch (e) {
-      setUpdateError(e);
+      setUpdateError(e instanceof Error ? e : undefined);
       setResponseErrors(parseResponseErrors(e, ['updateEventProposal']));
     } finally {
       setUpdatePromise(undefined);
@@ -102,7 +102,7 @@ function EventProposalFormInner({
       setSubmitPromise(promise);
       await promise;
     } catch (e) {
-      setSubmitError(e);
+      setSubmitError(e instanceof Error ? e : undefined);
     } finally {
       setSubmitPromise(undefined);
     }
diff --git a/app/javascript/EventsApp/EventCatalog/EventList/EventListFilterableFormItemDropdown.tsx b/app/javascript/EventsApp/EventCatalog/EventList/EventListFilterableFormItemDropdown.tsx
index 6ca5739ec6b..f1d01c10f52 100644
--- a/app/javascript/EventsApp/EventCatalog/EventList/EventListFilterableFormItemDropdown.tsx
+++ b/app/javascript/EventsApp/EventCatalog/EventList/EventListFilterableFormItemDropdown.tsx
@@ -1,4 +1,5 @@
 import { useMemo } from 'react';
+import * as React from 'react';
 import { useLocation } from 'react-router';
 
 import { DropdownMenu } from '../../../UIComponents/DropdownMenu';
@@ -54,7 +55,12 @@ function EventListFilterableFormItemDropdown({
       dropdownClassName="p-2"
       shouldAutoCloseOnNavigate={shouldAutoCloseOnNavigate}
     >
-      <FormItemFilter convention={convention} formItem={formItem} onChange={onChange} value={value} />
+      <FormItemFilter
+        convention={convention}
+        formItem={formItem}
+        onChange={onChange as React.Dispatch<React.SetStateAction<string[] | undefined | null>>}
+        value={value}
+      />
     </DropdownMenu>
   );
 }
diff --git a/app/javascript/EventsApp/EventPage/WithdrawMySignupModal.tsx b/app/javascript/EventsApp/EventPage/WithdrawMySignupModal.tsx
index 1c8a0488521..42d7f3d8c3b 100644
--- a/app/javascript/EventsApp/EventPage/WithdrawMySignupModal.tsx
+++ b/app/javascript/EventsApp/EventPage/WithdrawMySignupModal.tsx
@@ -45,7 +45,7 @@ export function WithdrawMySignupModal({ close, event, run, signup, signupRounds
       close();
     } catch (error) {
       setBusy(false);
-      setError(error);
+      setError(error instanceof Error ? error : undefined);
     }
   };
 
diff --git a/app/javascript/EventsApp/ScheduleGrid/ScheduleLayout/ColumnReservationSet.ts b/app/javascript/EventsApp/ScheduleGrid/ScheduleLayout/ColumnReservationSet.ts
index 5f3c2212a81..3e017283207 100644
--- a/app/javascript/EventsApp/ScheduleGrid/ScheduleLayout/ColumnReservationSet.ts
+++ b/app/javascript/EventsApp/ScheduleGrid/ScheduleLayout/ColumnReservationSet.ts
@@ -34,9 +34,9 @@ function compareReservationsForSort(a: ColumnReservation | null, b: ColumnReserv
 }
 
 class ColumnReservationSet {
-  reservations: (ColumnReservation | null)[];
+  reservations!: (ColumnReservation | null)[];
 
-  columnNumberByRunId: Map<string, number>;
+  columnNumberByRunId!: Map<string, number>;
 
   constructor() {
     this.clear();
diff --git a/app/javascript/EventsApp/SignupAdmin/$id/route.tsx b/app/javascript/EventsApp/SignupAdmin/$id/route.tsx
index 12598b701ac..40571affaaf 100644
--- a/app/javascript/EventsApp/SignupAdmin/$id/route.tsx
+++ b/app/javascript/EventsApp/SignupAdmin/$id/route.tsx
@@ -1,6 +1,6 @@
 import { useCallback, useContext } from 'react';
 import classNames from 'classnames';
-import { ActionFunction, data, Link, Outlet, useSubmit, RouterContextProvider } from 'react-router';
+import { ActionFunction, data, Link, Outlet, useParams, useSubmit, RouterContextProvider } from 'react-router';
 import { Trans, useTranslation } from 'react-i18next';
 import { TFunction } from 'i18next';
 import { DateTime } from 'luxon';
@@ -90,10 +90,6 @@ function getToggleCountedConfirmPrompt(signup: SignupFieldsFragment) {
   return getMakeCountedConfirmPrompt(signup);
 }
 
-export type EditSignupProps = {
-  teamMembersUrl: string;
-};
-
 export const action: ActionFunction<RouterContextProvider> = async ({ context, request, params: { id } }) => {
   const client = context.get(apolloClientContext);
   const formData = await request.formData();
@@ -107,8 +103,10 @@ export const action: ActionFunction<RouterContextProvider> = async ({ context, r
   return data(result.data);
 };
 
-function EditSignup({ teamMembersUrl }: EditSignupProps): React.JSX.Element {
+function EditSignup(): React.JSX.Element {
   const data = useSingleSignupLoader();
+  const { eventId } = useParams();
+  const teamMembersUrl = `/events/${eventId}/team_members`;
   const { timezoneName, ticketName } = useContext(AppRootContext);
   const confirm = useConfirm();
   const { t } = useTranslation();
diff --git a/app/javascript/FormAdmin/$id/edit/section/$sectionId/FormSectionEditorContent.tsx b/app/javascript/FormAdmin/$id/edit/section/$sectionId/FormSectionEditorContent.tsx
index ab53b394846..e4323a304f5 100644
--- a/app/javascript/FormAdmin/$id/edit/section/$sectionId/FormSectionEditorContent.tsx
+++ b/app/javascript/FormAdmin/$id/edit/section/$sectionId/FormSectionEditorContent.tsx
@@ -56,7 +56,7 @@ function FormSectionEditorContent(): React.JSX.Element {
           <div className="card-header">
             <InPlaceEditor
               className="d-flex align-items-start w-100"
-              value={currentSection.title}
+              value={currentSection.title ?? null}
               onChange={updateSectionTitle}
             >
               <h4 className="m-0">{currentSection.title}</h4>
diff --git a/app/javascript/FormAdmin/$id/edit/section/$sectionId/NewFormItemModal.tsx b/app/javascript/FormAdmin/$id/edit/section/$sectionId/NewFormItemModal.tsx
index 485b7e1a3e4..075e50087e9 100644
--- a/app/javascript/FormAdmin/$id/edit/section/$sectionId/NewFormItemModal.tsx
+++ b/app/javascript/FormAdmin/$id/edit/section/$sectionId/NewFormItemModal.tsx
@@ -236,7 +236,7 @@ function NewFormItemModal<FormType extends FormTypeDefinition>({
           <BootstrapFormSelect
             label="Item type"
             value={itemType || ''}
-            onValueChange={(newValue: TypedFormItem['item_type']) => setItemType(newValue)}
+            onValueChange={(newValue) => setItemType(newValue as TypedFormItem['item_type'])}
           >
             <option aria-hidden value="">
               {t('admin.forms.newFormItem.itemTypeSelectPlaceholder')}
diff --git a/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/FormItemEditorContent.tsx b/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/FormItemEditorContent.tsx
index 94cc50541ef..8534b69f3b9 100644
--- a/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/FormItemEditorContent.tsx
+++ b/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/FormItemEditorContent.tsx
@@ -17,23 +17,68 @@ function FormItemEditorContent(): React.JSX.Element {
 
   switch (formItem.item_type) {
     case 'age_restrictions':
-      return <AgeRestrictionsEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <AgeRestrictionsEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'date':
-      return <DateEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <DateEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'event_email':
-      return <EventEmailEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <EventEmailEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'free_text':
-      return <FreeTextEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <FreeTextEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'multiple_choice':
-      return <MultipleChoiceEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <MultipleChoiceEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'registration_policy':
-      return <RegistrationPolicyItemEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <RegistrationPolicyItemEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'static_text':
-      return <StaticTextEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <StaticTextEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'timeblock_preference':
-      return <TimeblockPreferenceEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <TimeblockPreferenceEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     case 'timespan':
-      return <TimespanEditor formItem={formItem} setFormItem={setFormItem} />;
+      return (
+        <TimespanEditor
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<typeof formItem>>}
+        />
+      );
     default:
       assertNever(formItem, true);
       errorReporting().warning(`Unknown form item type: ${formItem.item_type}`);
diff --git a/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/route.tsx b/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/route.tsx
index 07e78bca5a4..58de218359a 100644
--- a/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/route.tsx
+++ b/app/javascript/FormAdmin/$id/edit/section/$sectionId/item/$itemId/route.tsx
@@ -1,3 +1,4 @@
+import * as React from 'react';
 import { useContext, useMemo, useState, useCallback } from 'react';
 
 import { useApolloClient } from '@apollo/client/react';
@@ -84,7 +85,10 @@ function FormItemEditorLayout(): React.JSX.Element {
   );
   const [previewFormItem, setPreviewFormItem] = useState(() => formItemsById.get(initialFormItem?.id ?? ''));
   const refreshRenderedFormItem = useCallback(
-    async (newFormItem: FormEditorFormItem) => {
+    async (newFormItem: FormEditorFormItem | undefined) => {
+      if (!newFormItem) {
+        return;
+      }
       if (!currentSection?.id) {
         return;
       }
@@ -103,7 +107,7 @@ function FormItemEditorLayout(): React.JSX.Element {
         setPreviewFormItem(responseFormItem);
       }
     },
-    [apolloClient, currentSection?.id, form.id],
+    [apolloClient, currentSection, form.id],
   );
   const updateFetcher = useFetcher();
   const updateError = updateFetcher.data instanceof Error ? updateFetcher.data : undefined;
@@ -143,7 +147,7 @@ function FormItemEditorLayout(): React.JSX.Element {
       value={{
         disabled: updateInProgress,
         formItem,
-        setFormItem,
+        setFormItem: setFormItem as React.Dispatch<React.SetStateAction<FormEditorFormItem>>,
         standardItem,
         previewFormItem,
       }}
diff --git a/app/javascript/FormAdmin/FormItemTools.tsx b/app/javascript/FormAdmin/FormItemTools.tsx
index 23488389e95..91aed86eb31 100644
--- a/app/javascript/FormAdmin/FormItemTools.tsx
+++ b/app/javascript/FormAdmin/FormItemTools.tsx
@@ -1,3 +1,4 @@
+import * as React from 'react';
 import { useContext, useId, useRef, useState } from 'react';
 import { useFetcher, useNavigate, useParams } from 'react-router';
 
@@ -5,7 +6,7 @@ import { Modal } from 'react-bootstrap4-modal';
 import { useModal, MultipleChoiceInput, ErrorDisplay } from '@neinteractiveliterature/litform';
 
 import { FormItemEditorContext, FormEditorContext } from './FormEditorContexts';
-import CommonQuestionFields from './ItemEditors/CommonQuestionFields';
+import CommonQuestionFields, { CommonQuestionFieldsProps } from './ItemEditors/CommonQuestionFields';
 import useCollapse from '../NavigationBar/useCollapse';
 import humanize from '../humanize';
 
@@ -173,7 +174,10 @@ function FormItemTools({ saveFormItem }: FormItemToolsProps): React.JSX.Element
         <i className={collapsed ? 'bi-caret-right' : 'bi-caret-down'} /> Tools
       </button>
       <div id={collapseId} className={`${collapseClassName} d-lg-block`} ref={collapseRef} {...otherCollapseProps}>
-        <CommonQuestionFields formItem={formItem} setFormItem={setFormItem} />
+        <CommonQuestionFields
+          formItem={formItem}
+          setFormItem={setFormItem as React.Dispatch<React.SetStateAction<CommonQuestionFieldsProps['formItem']>>}
+        />
         <button className="btn btn-secondary mt-2" type="button" onClick={moveModal.open}>
           Move to another section
         </button>
diff --git a/app/javascript/FormAdmin/FormItemUtils.ts b/app/javascript/FormAdmin/FormItemUtils.ts
index 2636038e64f..01cacd7907e 100644
--- a/app/javascript/FormAdmin/FormItemUtils.ts
+++ b/app/javascript/FormAdmin/FormItemUtils.ts
@@ -1,3 +1,4 @@
+import * as React from 'react';
 import { v4 as uuidv4 } from 'uuid';
 import { ApolloCache } from '@apollo/client';
 import { assertNever } from 'assert-never';
@@ -417,7 +418,7 @@ export function formItemPropertyUpdater<
   PropertiesType extends Record<string, unknown>,
   FormItemType extends ParsedFormItem<PropertiesType, unknown>,
   PropertyName extends keyof PropertiesType,
->(property: PropertyName, onChange: (mutator: (prevFormItem: FormItemType) => FormItemType) => void) {
+>(property: PropertyName, onChange: React.Dispatch<React.SetStateAction<FormItemType>>) {
   return (newValue: PropertiesType[PropertyName]): void =>
     onChange((prevFormItem) => {
       const newFormItem = {
diff --git a/app/javascript/FormAdmin/ItemEditors/CommonQuestionFields.tsx b/app/javascript/FormAdmin/ItemEditors/CommonQuestionFields.tsx
index 5efcd2085e7..a9ae7e40929 100644
--- a/app/javascript/FormAdmin/ItemEditors/CommonQuestionFields.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/CommonQuestionFields.tsx
@@ -1,3 +1,4 @@
+import * as React from 'react';
 import { useContext } from 'react';
 import { BootstrapFormInput, BooleanInput, useModal, MultipleChoiceInput } from '@neinteractiveliterature/litform';
 import { useTranslation } from 'react-i18next';
@@ -87,7 +88,7 @@ function CommonQuestionFields({ formItem, setFormItem }: CommonQuestionFieldsPro
             value={!!formItem.properties.required}
             onChange={formItemPropertyUpdater<CommonQuestionProperties, QuestionFormItem, 'required'>(
               'required',
-              setFormItem,
+              setFormItem as React.Dispatch<React.SetStateAction<QuestionFormItem>>,
             )}
           />
         ))}
diff --git a/app/javascript/FormAdmin/ItemEditors/DefaultAnswerModal.tsx b/app/javascript/FormAdmin/ItemEditors/DefaultAnswerModal.tsx
index 71f5c4cc101..15845567107 100644
--- a/app/javascript/FormAdmin/ItemEditors/DefaultAnswerModal.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/DefaultAnswerModal.tsx
@@ -20,7 +20,8 @@ function DefaultAnswerModal<FormItemType extends TypedFormItem>({
   const { convention, formTypeIdentifier } = useContext(FormEditorContext);
   const { previewFormItem } = useContext(FormItemEditorContext);
   const [defaultValue, setDefaultValue] = useState<FormItemType['default_value']>(formItem.default_value);
-  const inputChanged = (identifier: string, newValue: typeof defaultValue) => setDefaultValue(newValue);
+  const inputChanged = (identifier: string, newValue: unknown) =>
+    setDefaultValue(newValue as FormItemType['default_value']);
 
   const setDefaultAnswer = () => {
     setFormItem({ ...formItem, default_value: defaultValue });
diff --git a/app/javascript/FormAdmin/ItemEditors/EventEmailEditor.tsx b/app/javascript/FormAdmin/ItemEditors/EventEmailEditor.tsx
index 902cd53b212..9f4027d02c3 100644
--- a/app/javascript/FormAdmin/ItemEditors/EventEmailEditor.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/EventEmailEditor.tsx
@@ -1,10 +1,10 @@
-import CommonQuestionFields from './CommonQuestionFields';
+import CommonQuestionFields, { CommonQuestionFieldsProps } from './CommonQuestionFields';
 import { FormItemEditorProps } from '../FormItemEditorProps';
 import { EventEmailFormItem } from '../FormItemUtils';
 
 export type EventEmailEditorProps = FormItemEditorProps<EventEmailFormItem>;
 function EventEmailEditor(props: EventEmailEditorProps): React.JSX.Element {
-  return <CommonQuestionFields {...props} />;
+  return <CommonQuestionFields {...(props as CommonQuestionFieldsProps)} />;
 }
 
 export default EventEmailEditor;
diff --git a/app/javascript/FormAdmin/ItemEditors/FreeTextEditor.tsx b/app/javascript/FormAdmin/ItemEditors/FreeTextEditor.tsx
index aa242da4b6c..bdb14c8fbf3 100644
--- a/app/javascript/FormAdmin/ItemEditors/FreeTextEditor.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/FreeTextEditor.tsx
@@ -2,7 +2,7 @@ import { useContext, useId } from 'react';
 import { BootstrapFormInput, BootstrapFormSelect, parseIntOrNull } from '@neinteractiveliterature/litform';
 
 import LiquidInput from '../../BuiltInFormControls/LiquidInput';
-import { formItemPropertyUpdater, FreeTextFormItem } from '../FormItemUtils';
+import { formItemPropertyUpdater, FreeTextFormItem, FreeTextProperties } from '../FormItemUtils';
 import { FormItemEditorContext } from '../FormEditorContexts';
 import { FormItemEditorProps } from '../FormItemEditorProps';
 
@@ -12,13 +12,16 @@ function FreeTextEditor({ formItem, setFormItem }: FreeTextEditorProps): React.J
   const captionInputId = useId();
   const responseFormat =
     formItem.properties.format === 'markdown' ? 'markdown' : formItem.properties.free_text_type || 'text';
-  const setResponseFormat = (newResponseFormat: typeof responseFormat) => {
+  const setResponseFormat = (newResponseFormat: string) => {
     setFormItem((prevFormItem) => ({
       ...prevFormItem,
       properties: {
         ...prevFormItem.properties,
         format: newResponseFormat === 'markdown' ? 'markdown' : 'text',
-        free_text_type: newResponseFormat === 'markdown' ? null : newResponseFormat,
+        free_text_type:
+          newResponseFormat === 'markdown'
+            ? null
+            : (newResponseFormat as NonNullable<FreeTextProperties['free_text_type']>),
       },
     }));
   };
diff --git a/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetModal.tsx b/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetModal.tsx
index 32aa1682ef5..2d08dc129b7 100644
--- a/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetModal.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetModal.tsx
@@ -7,7 +7,7 @@ import { RegistrationPolicyPreset } from '../FormItemUtils';
 
 export type RegistrationPolicyItemEditorPresetModalProps = {
   initialPreset: RegistrationPolicyPreset;
-  onChange: React.Dispatch<React.SetStateAction<RegistrationPolicyPreset>>;
+  onChange: (preset: RegistrationPolicyPreset) => void;
   visible: boolean;
   close: () => void;
 };
diff --git a/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetRow.tsx b/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetRow.tsx
index fd41a6c7aae..5dfeed4a2ec 100644
--- a/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetRow.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/RegistrationPolicyItemEditorPresetRow.tsx
@@ -11,7 +11,7 @@ import { getSortableStyle } from '../../SortableUtils';
 type PresetWithId = WithGeneratedId<RegistrationPolicyPreset, string>;
 
 function usePresetPropertyUpdater<PropertyName extends keyof RegistrationPolicyPreset>(
-  onChange: (generatedId: string, preset: React.SetStateAction<PresetWithId>) => void,
+  onChange: (generatedId: string, updater: (prev: PresetWithId) => PresetWithId) => void,
   generatedId: string,
   property: PropertyName,
 ) {
@@ -28,7 +28,7 @@ function usePresetPropertyUpdater<PropertyName extends keyof RegistrationPolicyP
 export type RegistrationPolicyItemEditorPresetRowProps = {
   preset: PresetWithId;
   deletePreset: (generatedId: string) => void;
-  onChange: (generatedId: string, preset: React.SetStateAction<PresetWithId>) => void;
+  onChange: (generatedId: string, updater: (prev: PresetWithId) => PresetWithId) => void;
 };
 
 function RegistrationPolicyItemEditorPresetRow({
@@ -43,7 +43,8 @@ function RegistrationPolicyItemEditorPresetRow({
   });
 
   const presetChanged = useCallback(
-    (newPreset: PresetWithId) => onChange(preset.generatedId, () => newPreset),
+    (newPreset: RegistrationPolicyPreset) =>
+      onChange(preset.generatedId, () => ({ ...newPreset, generatedId: preset.generatedId })),
     [onChange, preset.generatedId],
   );
   const presetNameChanged = usePresetPropertyUpdater(onChange, preset.generatedId, 'name');
diff --git a/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRow.tsx b/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRow.tsx
index bc383abc124..e88917d5d0b 100644
--- a/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRow.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRow.tsx
@@ -13,7 +13,10 @@ import { getTimeblockTimespanForDisplay } from '../../FormPresenter/TimeblockUti
 import { getSortableStyle } from '../../SortableUtils';
 
 function useTimeblockPropertyUpdater<PropertyName extends keyof TimeblockDefinition>(
-  onChange: (generatedId: string, updater: React.SetStateAction<TimeblockDefinition>) => void,
+  onChange: (
+    generatedId: string,
+    updater: (prev: WithGeneratedId<TimeblockDefinition, string>) => WithGeneratedId<TimeblockDefinition, string>,
+  ) => void,
   generatedId: string,
   property: PropertyName,
 ) {
@@ -29,7 +32,10 @@ function useTimeblockPropertyUpdater<PropertyName extends keyof TimeblockDefinit
 
 export type TimeblockPreferenceEditorTimeblockRowProps = {
   timeblock: WithGeneratedId<TimeblockDefinition, string>;
-  onChange: (generatedId: string, updater: React.SetStateAction<TimeblockDefinition>) => void;
+  onChange: (
+    generatedId: string,
+    updater: (prev: WithGeneratedId<TimeblockDefinition, string>) => WithGeneratedId<TimeblockDefinition, string>,
+  ) => void;
   deleteTimeblock: (generatedId: string) => void;
 };
 
@@ -64,7 +70,7 @@ function TimeblockPreferenceEditorTimeblockRow({
     try {
       getTimeblockTimespanForDisplay(timeblock);
     } catch (e) {
-      return e.message;
+      return e instanceof Error ? e.message : String(e);
     }
 
     return null;
diff --git a/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRowDragOverlay.tsx b/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRowDragOverlay.tsx
index 0f997a0d2b7..1e93523aa9d 100644
--- a/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRowDragOverlay.tsx
+++ b/app/javascript/FormAdmin/ItemEditors/TimeblockPreferenceEditorTimeblockRowDragOverlay.tsx
@@ -34,7 +34,7 @@ function TimeblockPreferenceEditorTimeblockRowDragOverlay({
     try {
       getTimeblockTimespanForDisplay(timeblock);
     } catch (e) {
-      return e.message;
+      return e instanceof Error ? e.message : String(e);
     }
 
     return null;
diff --git a/app/javascript/FormAdmin/NewFormModal.tsx b/app/javascript/FormAdmin/NewFormModal.tsx
index bafbc86ab72..11e6318f7c1 100644
--- a/app/javascript/FormAdmin/NewFormModal.tsx
+++ b/app/javascript/FormAdmin/NewFormModal.tsx
@@ -44,7 +44,7 @@ function NewFormModal({ visible, close }: NewFormModalProps): React.JSX.Element
             disabled={inProgress}
           >
             <option value="">{t('admin.forms.newForm.formTypePlaceholder')}</option>
-            {Object.keys(FormTypes).map((value: keyof typeof FormTypes) => (
+            {(Object.keys(FormTypes) as (keyof typeof FormTypes)[]).map((value) => (
               <option value={value} key={value}>
                 {t(`admin.forms.formTypes.${value}`)}
               </option>
diff --git a/app/javascript/FormPresenter/ItemChangeDisplays/FormItemChangeDisplay.tsx b/app/javascript/FormPresenter/ItemChangeDisplays/FormItemChangeDisplay.tsx
index 404d971df04..913f12a3911 100644
--- a/app/javascript/FormPresenter/ItemChangeDisplays/FormItemChangeDisplay.tsx
+++ b/app/javascript/FormPresenter/ItemChangeDisplays/FormItemChangeDisplay.tsx
@@ -78,7 +78,12 @@ function FormItemChangeDisplay<FormItemType extends TypedFormItem>({
       return <TextDiffDisplay before={stringifiedPreviousValue} after={stringifiedNewValue} />;
     }
 
-    return <ObjectDiffDisplay before={change.previous_value} after={change.new_value} />;
+    return (
+      <ObjectDiffDisplay
+        before={change.previous_value as Record<string, { toString(): string } | null | undefined>}
+        after={change.new_value as Record<string, { toString(): string } | null | undefined>}
+      />
+    );
   }
 
   switch (formItem.item_type) {
diff --git a/app/javascript/FormPresenter/ItemDisplays/MultipleChoiceItemDisplay.tsx b/app/javascript/FormPresenter/ItemDisplays/MultipleChoiceItemDisplay.tsx
index 3f4a7d419d4..674ce3a6b4e 100644
--- a/app/javascript/FormPresenter/ItemDisplays/MultipleChoiceItemDisplay.tsx
+++ b/app/javascript/FormPresenter/ItemDisplays/MultipleChoiceItemDisplay.tsx
@@ -25,7 +25,8 @@ function MultipleChoiceItemDisplay({
   value: uncastValue,
 }: MultipleChoiceItemDisplayProps): React.JSX.Element {
   const value = castValue(uncastValue);
-  const isValueOther = (v: string) => !formItem.rendered_properties.choices.some((choice) => choice.value === v);
+  const isValueOther = (v: CastedValue): v is string =>
+    typeof v === 'string' && !formItem.rendered_properties.choices.some((choice) => choice.value === v);
 
   if (Array.isArray(value)) {
     const selectedChoiceLabels = formItem.rendered_properties.choices
diff --git a/app/javascript/MyProfile/MyProfileForm.tsx b/app/javascript/MyProfile/MyProfileForm.tsx
index 8271093ac47..63e16c37fb1 100644
--- a/app/javascript/MyProfile/MyProfileForm.tsx
+++ b/app/javascript/MyProfile/MyProfileForm.tsx
@@ -1,5 +1,14 @@
 import { useState, useContext } from 'react';
-import { ActionFunction, Link, LoaderFunction, SubmitTarget, useFetcher, useLoaderData, useMatch, RouterContextProvider } from 'react-router';
+import {
+  ActionFunction,
+  Link,
+  LoaderFunction,
+  SubmitTarget,
+  useFetcher,
+  useLoaderData,
+  useMatch,
+  RouterContextProvider,
+} from 'react-router';
 import md5 from 'md5';
 import { useTranslation, Trans } from 'react-i18next';
 import { BooleanInput, LoadingIndicator } from '@neinteractiveliterature/litform';
@@ -131,7 +140,7 @@ function MyProfileForm() {
     currentUserWriterRole: initialUserConProfile.current_user_form_item_writer_role,
     response: userConProfile,
     responseErrors,
-    responseValuesChanged,
+    responseValuesChanged: responseValuesChanged as (newValues: Record<string, unknown>) => void,
   };
 
   return (
diff --git a/app/javascript/NavigationBar/SiteSearch.tsx b/app/javascript/NavigationBar/SiteSearch.tsx
index 9a20b080af6..e88ff6b4b39 100644
--- a/app/javascript/NavigationBar/SiteSearch.tsx
+++ b/app/javascript/NavigationBar/SiteSearch.tsx
@@ -193,7 +193,8 @@ function SiteSearch({ visible, setVisible, visibilityChangeComplete }: SiteSearc
   }, [keyDownListener]);
 
   const optionSelected = useCallback(
-    (entry: SiteSearchOptionType) => {
+    (entry: SiteSearchOptionType | null) => {
+      if (!entry) return;
       const { model } = entry;
       if (model.__typename === 'Page') {
         navigate(`/pages/${(model as { slug: string }).slug}`);
diff --git a/app/javascript/OrganizationAdmin/useOrganizationRoleForm.tsx b/app/javascript/OrganizationAdmin/useOrganizationRoleForm.tsx
index 428f2f239ad..0fef7ecb194 100644
--- a/app/javascript/OrganizationAdmin/useOrganizationRoleForm.tsx
+++ b/app/javascript/OrganizationAdmin/useOrganizationRoleForm.tsx
@@ -7,7 +7,7 @@ import UserSelect from '../BuiltInFormControls/UserSelect';
 import PermissionNames from '../../../config/permission_names.json';
 import PermissionsTableInput from '../Permissions/PermissionsTableInput';
 import { OrganizationAdminOrganizationsQueryData } from './queries.generated';
-import { PermissionWithId } from '../Permissions/usePermissionsChangeSet';
+import { PermissionWithId, UsePermissionsChangeSetOptions } from '../Permissions/usePermissionsChangeSet';
 
 const OrganizationRolePermissionNames = flatMap(
   PermissionNames.filter((permissionNameGroup) => permissionNameGroup.role_type === 'OrganizationRole'),
@@ -69,7 +69,7 @@ export default function useOrganizationRoleForm(initialOrganizationRole: Organiz
           rows={[initialOrganizationRole]}
           model={undefined}
           changeSet={permissionsChangeSet}
-          add={addPermission}
+          add={addPermission as unknown as UsePermissionsChangeSetOptions['add']}
           remove={removePermission}
           formatRowHeader={() => 'Permitted?'}
         />
diff --git a/app/javascript/Permissions/PermissionsTableInput.tsx b/app/javascript/Permissions/PermissionsTableInput.tsx
index 39a13a4b2ed..40637b13c48 100644
--- a/app/javascript/Permissions/PermissionsTableInput.tsx
+++ b/app/javascript/Permissions/PermissionsTableInput.tsx
@@ -50,6 +50,12 @@ type PermissionsTableInputProps<RowsType extends BaseRowType[]> = RowsType exten
     ? PermissionsTableInputRoleRowProps<RowsType>
     : never;
 
+function PermissionsTableInput<RowsType extends (Pick<PermissionedModel, '__typename'> & { id: string })[]>(
+  props: PermissionsTableInputModelRowProps<RowsType>,
+): React.JSX.Element;
+function PermissionsTableInput<RowsType extends (Pick<PermissionedRole, '__typename'> & { id: string })[]>(
+  props: PermissionsTableInputRoleRowProps<RowsType>,
+): React.JSX.Element;
 function PermissionsTableInput<RowsType extends BaseRowType[]>(
   props: PermissionsTableInputProps<RowsType>,
 ): React.JSX.Element {
diff --git a/app/javascript/RootSiteAdmin/EditRootSite.tsx b/app/javascript/RootSiteAdmin/EditRootSite.tsx
index 1d8898f0ce1..f9aefe0ba57 100644
--- a/app/javascript/RootSiteAdmin/EditRootSite.tsx
+++ b/app/javascript/RootSiteAdmin/EditRootSite.tsx
@@ -74,7 +74,9 @@ function EditRootSite() {
 
   useEffect(() => {
     if (navigation.state === 'idle' && actionData != null && !error) {
+      // eslint-disable-next-line react-hooks/set-state-in-effect
       setSuccess(true);
+       
       setEdited(false);
     }
   }, [actionData, error, navigation.state]);
@@ -100,7 +102,7 @@ function EditRootSite() {
         getOptionValue={(option) => option.id.toString()}
         getOptionLabel={(option) => option.name ?? ''}
         options={data.rootSite.cmsLayouts}
-        onChange={(newValue: (typeof data)['rootSite']['cmsLayouts'][0]) => setDefaultLayout(newValue)}
+        onChange={(newValue) => newValue && setDefaultLayout(newValue)}
         isDisabled={updateInProgress}
       />
 
@@ -112,7 +114,7 @@ function EditRootSite() {
         getOptionValue={(option) => option.id.toString()}
         getOptionLabel={(option) => option.name ?? ''}
         options={data.rootSite.cmsPages}
-        onChange={(newValue: (typeof data)['rootSite']['rootPage']) => setRootPage(newValue)}
+        onChange={(newValue) => newValue && setRootPage(newValue)}
         isDisabled={updateInProgress}
       />
 
diff --git a/app/javascript/RootSiteConventionsAdmin/NewConventionModal.tsx b/app/javascript/RootSiteConventionsAdmin/NewConventionModal.tsx
index 7956b41cefe..1a87fdc3af5 100644
--- a/app/javascript/RootSiteConventionsAdmin/NewConventionModal.tsx
+++ b/app/javascript/RootSiteConventionsAdmin/NewConventionModal.tsx
@@ -1,4 +1,5 @@
 import { useState, useMemo } from 'react';
+import * as React from 'react';
 import { Modal } from 'react-bootstrap4-modal';
 
 import { Link, useFetcher } from 'react-router';
@@ -150,7 +151,10 @@ export default function NewConventionModal({ data, cloneConvention }: NewConvent
           isClearable
         />
 
-        <ConventionLanguageInput value={convention.language} onChange={setLanguage} />
+        <ConventionLanguageInput
+          value={convention.language}
+          onChange={setLanguage as React.Dispatch<React.SetStateAction<string | null | undefined>>}
+        />
 
         <MultipleChoiceInput
           caption="Date and time display mode"
@@ -195,7 +199,7 @@ export default function NewConventionModal({ data, cloneConvention }: NewConvent
             label="Initial CMS content set"
             options={CMS_CONTENT_SET_OPTIONS}
             value={cmsContentSet}
-            onChange={(newValue: (typeof CMS_CONTENT_SET_OPTIONS)[0]) => setCmsContentSet(newValue)}
+            onChange={(newValue) => setCmsContentSet(newValue)}
             getOptionLabel={(option) => option.label}
             getOptionValue={(option) => option.name}
           />
diff --git a/app/javascript/SignupModeration/CreateSignup.tsx b/app/javascript/SignupModeration/CreateSignup.tsx
index 829afaa039a..7166e419476 100644
--- a/app/javascript/SignupModeration/CreateSignup.tsx
+++ b/app/javascript/SignupModeration/CreateSignup.tsx
@@ -32,7 +32,7 @@ function CreateSignup(): React.JSX.Element {
           <EventSelect
             id={id}
             value={event}
-            onChange={(newEvent: EventType) => setEvent(newEvent)}
+            onChange={(newEvent) => setEvent(newEvent ?? undefined)}
             eventsQuery={CreateSignupEventsQueryDocument}
           />
         )}
@@ -43,7 +43,7 @@ function CreateSignup(): React.JSX.Element {
           <UserConProfileSelect
             id={id}
             value={userConProfile}
-            onChange={(newUserConProfile: UserConProfileType) => setUserConProfile(newUserConProfile)}
+            onChange={(newUserConProfile) => setUserConProfile(newUserConProfile ?? undefined)}
           />
         )}
       </FormGroupWithLabel>
diff --git a/app/javascript/SignupModeration/RankedChoiceQueue.tsx b/app/javascript/SignupModeration/RankedChoiceQueue.tsx
index e74df67eee6..7c391e53eb7 100644
--- a/app/javascript/SignupModeration/RankedChoiceQueue.tsx
+++ b/app/javascript/SignupModeration/RankedChoiceQueue.tsx
@@ -25,7 +25,7 @@ function RankedChoiceQueue() {
           <UserConProfileSelect
             id={id}
             value={userConProfile}
-            onChange={(newUserConProfile: UserConProfileType) => setUserConProfile(newUserConProfile)}
+            onChange={(newUserConProfile) => setUserConProfile(newUserConProfile ?? undefined)}
           />
         )}
       </FormGroupWithLabel>
diff --git a/app/javascript/StaffPositionAdmin/EditStaffPositionPermissions.tsx b/app/javascript/StaffPositionAdmin/EditStaffPositionPermissions.tsx
index d832aef21ae..3dff8b399e1 100644
--- a/app/javascript/StaffPositionAdmin/EditStaffPositionPermissions.tsx
+++ b/app/javascript/StaffPositionAdmin/EditStaffPositionPermissions.tsx
@@ -1,4 +1,11 @@
-import { ActionFunction, LoaderFunction, redirect, useFetcher, useLoaderData, RouterContextProvider } from 'react-router';
+import {
+  ActionFunction,
+  LoaderFunction,
+  redirect,
+  useFetcher,
+  useLoaderData,
+  RouterContextProvider,
+} from 'react-router';
 import { useTabs, TabList, TabBody, notEmpty, ErrorDisplay } from '@neinteractiveliterature/litform';
 
 import { getEventCategoryStyles } from '../EventsApp/ScheduleGrid/StylingUtils';
@@ -9,7 +16,7 @@ import usePageTitle from '../usePageTitle';
 import { getPermissionNamesForModelType, buildPermissionInput } from '../Permissions/PermissionUtils';
 import { PermissionedModelTypeIndicator } from '../graphqlTypes.generated';
 import { StaffPositionsQueryData, StaffPositionsQueryDocument } from './queries.generated';
-import { PermissionWithId } from '../Permissions/usePermissionsChangeSet';
+import { PermissionWithId, UsePermissionsChangeSetOptions } from '../Permissions/usePermissionsChangeSet';
 import { apolloClientContext } from 'AppContexts';
 import {
   UpdateStaffPositionPermissionsDocument,
@@ -76,7 +83,7 @@ function EditStaffPositionPermissions() {
           role={staffPosition}
           model={convention}
           changeSet={conventionChangeSet}
-          add={conventionAdd}
+          add={conventionAdd as unknown as UsePermissionsChangeSetOptions['add']}
           remove={conventionRemove}
           reset={conventionReset}
           header={convention.name}
@@ -94,7 +101,7 @@ function EditStaffPositionPermissions() {
           role={staffPosition}
           rows={convention.event_categories}
           changeSet={eventCategoriesChangeSet}
-          add={eventCategoriesAdd}
+          add={eventCategoriesAdd as unknown as UsePermissionsChangeSetOptions['add']}
           remove={eventCategoriesRemove}
           rowsHeader="Event Category"
           formatRowHeader={(eventCategory) => (
@@ -116,7 +123,7 @@ function EditStaffPositionPermissions() {
           role={staffPosition}
           rows={convention.cmsContentGroups}
           changeSet={contentGroupsChangeSet}
-          add={contentGroupsAdd}
+          add={contentGroupsAdd as unknown as UsePermissionsChangeSetOptions['add']}
           remove={contentGroupsRemove}
           rowsHeader="CMS Content Group"
           formatRowHeader={(contentGroup) => contentGroup.name}
diff --git a/app/javascript/StaffPositionAdmin/StaffPositionsTable.tsx b/app/javascript/StaffPositionAdmin/StaffPositionsTable.tsx
index 9cab48d7e4a..a0a48126032 100644
--- a/app/javascript/StaffPositionAdmin/StaffPositionsTable.tsx
+++ b/app/javascript/StaffPositionAdmin/StaffPositionsTable.tsx
@@ -57,7 +57,7 @@ function describePermissionAbilities(modelPermissions: Pick<PolymorphicPermissio
   const typename = modelPermissions[0].model.__typename;
   const permissionNameGroups = PermissionNames.filter((group) => group.model_type === typename);
   const permissionNamesForType = flatMap(permissionNameGroups, (group) => group.permissions);
-  const abilities = permissionNamesForType.reduce((acc, { permission, name }) => {
+  const abilities = permissionNamesForType.reduce<string[]>((acc, { permission, name }) => {
     if (modelPermissions.some((modelPermission) => modelPermission.permission === permission)) {
       return [...acc, name];
     }
diff --git a/app/javascript/Store/CouponAdmin/CouponForm.tsx b/app/javascript/Store/CouponAdmin/CouponForm.tsx
index 767e6fc4df3..ddc6c51166f 100644
--- a/app/javascript/Store/CouponAdmin/CouponForm.tsx
+++ b/app/javascript/Store/CouponAdmin/CouponForm.tsx
@@ -106,7 +106,12 @@ function CouponForm<T extends Omit<AdminCouponFieldsFragment, 'id'>>({
               id={id}
               value={value.fixed_amount}
               onChange={(newFixedAmount) => {
-                setFixedAmount(newFixedAmount ?? buildBlankFixedAmount(defaultCurrencyCode));
+                const amount = typeof newFixedAmount === 'function' ? undefined : newFixedAmount;
+                setFixedAmount(
+                  (amount ?? buildBlankFixedAmount(defaultCurrencyCode)) as unknown as Parameters<
+                    typeof setFixedAmount
+                  >[0],
+                );
               }}
             />
           )}
diff --git a/app/javascript/Store/OrderAdmin/AdminOrderEntriesTable.tsx b/app/javascript/Store/OrderAdmin/AdminOrderEntriesTable.tsx
index d9b88f4f6ea..245e0be0fdb 100644
--- a/app/javascript/Store/OrderAdmin/AdminOrderEntriesTable.tsx
+++ b/app/javascript/Store/OrderAdmin/AdminOrderEntriesTable.tsx
@@ -252,7 +252,7 @@ function AdminOrderEntriesTable<
             <td>
               <ProductSelect
                 value={addingItem.product}
-                onChange={setAddingItemProduct}
+                onChange={setAddingItemProduct as unknown as Parameters<typeof ProductSelect>[0]['onChange']}
                 isClearable
                 isDisabled={createInProgress}
               />
@@ -260,7 +260,7 @@ function AdminOrderEntriesTable<
                 <Select
                   options={addingItem.product?.product_variants ?? []}
                   value={addingItem.product_variant}
-                  onChange={setAddingItemProductVariant}
+                  onChange={(variant) => setAddingItemProductVariant(variant ?? null)}
                   getOptionValue={(variant) => variant?.id?.toString() ?? ''}
                   getOptionLabel={(variant) => variant?.name ?? ''}
                   isClearable
diff --git a/app/javascript/Store/OrderPaymentModal.tsx b/app/javascript/Store/OrderPaymentModal.tsx
index a85cbd85000..18514884841 100644
--- a/app/javascript/Store/OrderPaymentModal.tsx
+++ b/app/javascript/Store/OrderPaymentModal.tsx
@@ -66,7 +66,7 @@ export function OrderPaymentModalContents({ onCancel, onComplete, onError, order
         await submit({ totalPrice, paymentMode, orderId: order.id, submitOrder });
         onComplete();
       } catch (error) {
-        onError(error);
+        onError(error as Error);
       }
     },
     { suppressError: true },
@@ -173,11 +173,12 @@ function OrderPaymentModal({
         result.data?.convention.my_profile?.current_pending_order?.payment_intent_client_secret,
       );
     } catch (error) {
-      onErrorRef.current(error);
+      onErrorRef.current(error as Error);
     }
   }, [client]);
 
   useEffect(() => {
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setPaymentIntentClientSecret(undefined);
 
     if (paymentIntentClientSecretNeeded) {
diff --git a/app/javascript/Store/ProductAdmin/AdminProductVariantsTable.tsx b/app/javascript/Store/ProductAdmin/AdminProductVariantsTable.tsx
index 54291ce806b..52fd42b9106 100644
--- a/app/javascript/Store/ProductAdmin/AdminProductVariantsTable.tsx
+++ b/app/javascript/Store/ProductAdmin/AdminProductVariantsTable.tsx
@@ -37,7 +37,7 @@ type AdminProductVariantsTableCommonProps = {
 
 type AdminProductVariantsTableEditingProps = AdminProductVariantsTableCommonProps & {
   editing: boolean;
-  onChange: React.Dispatch<EditingVariant[]>;
+  onChange: React.Dispatch<React.SetStateAction<EditingVariant[]>>;
   deleteVariant: React.Dispatch<string>;
 };
 
@@ -124,11 +124,13 @@ function AdminProductVariantsTable(props: AdminProductVariantsTableProps): React
 
   const { draggingItem, ...sortableHandlers } = useBasicSortableHandlers(
     useCallback(
-      (id: string) => sortedVariants.find((variant) => getRealOrGeneratedId(variant).toString() === id),
+      (id: string | number) =>
+        sortedVariants.find((variant) => getRealOrGeneratedId(variant).toString() === id.toString()),
       [sortedVariants],
     ),
     useCallback(
-      (id: string) => sortedVariants.findIndex((variant) => getRealOrGeneratedId(variant).toString() === id),
+      (id: string | number) =>
+        sortedVariants.findIndex((variant) => getRealOrGeneratedId(variant).toString() === id.toString()),
       [sortedVariants],
     ),
     moveVariant,
diff --git a/app/javascript/Store/ProductAdmin/EditPricingStructureModal.tsx b/app/javascript/Store/ProductAdmin/EditPricingStructureModal.tsx
index eef99695c02..d157f3ffa9a 100644
--- a/app/javascript/Store/ProductAdmin/EditPricingStructureModal.tsx
+++ b/app/javascript/Store/ProductAdmin/EditPricingStructureModal.tsx
@@ -9,7 +9,7 @@ import PricingStructureForm, { PricingStructureFormProps } from './PricingStruct
 import { useTranslation } from 'react-i18next';
 import { ModalData } from '@neinteractiveliterature/litform';
 
-type EditingPricingStructure = PricingStructureFormProps['pricingStructure'];
+type EditingPricingStructure = NonNullable<PricingStructureFormProps['pricingStructure']>;
 
 export const PRICING_STRATEGIES = EnumTypes.PricingStrategy.enumValues.map(({ name, description }) => ({
   value: name,
@@ -35,15 +35,23 @@ export const PricingStructureModalContext = React.createContext<PricingStructure
   setState: () => {},
 });
 
-export const buildScheduledMoneyValueInput = (value: Money | null | undefined, onChange: React.Dispatch<Money>) => (
-  <MoneyInput value={value} onChange={onChange} />
+export const buildScheduledMoneyValueInput = (
+  value: Money | null | undefined,
+  onChange: React.Dispatch<Money | undefined>,
+) => (
+  <MoneyInput
+    value={value}
+    onChange={(newValue) => onChange(typeof newValue === 'function' ? newValue(value ?? undefined) : newValue)}
+  />
 );
 
 export type EditPricingStructureModalProps = Pick<PricingStructureModalContextValue, 'visible' | 'state' | 'close'>;
 
 function EditPricingStructureModal({ visible, state, close }: EditPricingStructureModalProps): React.JSX.Element {
   const { t } = useTranslation();
-  const [pricingStructure, setPricingStructure] = useState(state?.value);
+  const [pricingStructure, setPricingStructure] = useState<EditingPricingStructure | undefined>(
+    state?.value ?? undefined,
+  );
 
   const okClicked = () => {
     if (pricingStructure && state?.onChange) {
diff --git a/app/javascript/Store/ProductAdmin/EditProductForm.tsx b/app/javascript/Store/ProductAdmin/EditProductForm.tsx
index 6764b3a063b..5cf645da689 100644
--- a/app/javascript/Store/ProductAdmin/EditProductForm.tsx
+++ b/app/javascript/Store/ProductAdmin/EditProductForm.tsx
@@ -14,7 +14,7 @@ import { WithRealOrGeneratedId } from '../../GeneratedIdUtils';
 import { PricingStrategy } from '../../graphqlTypes.generated';
 import AdminProductVariantsTable from './AdminProductVariantsTable';
 import { EditingProductBase } from './EditingProductTypes';
-import PricingStructureForm from './PricingStructureForm';
+import PricingStructureForm, { PricingStructureFormProps } from './PricingStructureForm';
 import { AdminProductsQueryData } from './queries.generated';
 
 export type EditProductFormProps<ProductType extends WithRealOrGeneratedId<EditingProductBase, string>> = {
@@ -198,7 +198,7 @@ export default function EditProductForm<ProductType extends WithRealOrGeneratedI
                   },
                 }
               }
-              setPricingStructure={setPricingStructure}
+              setPricingStructure={setPricingStructure as unknown as PricingStructureFormProps['setPricingStructure']}
             />
           </div>
         </section>
diff --git a/app/javascript/Store/ProductAdmin/PricingStructureInput.tsx b/app/javascript/Store/ProductAdmin/PricingStructureInput.tsx
index abfa65df859..ea768ab6754 100644
--- a/app/javascript/Store/ProductAdmin/PricingStructureInput.tsx
+++ b/app/javascript/Store/ProductAdmin/PricingStructureInput.tsx
@@ -1,7 +1,7 @@
 import { useContext } from 'react';
 import * as React from 'react';
 
-import { PricingStructureModalContext } from './EditPricingStructureModal';
+import { PricingStructureModalContext, PricingStructureModalState } from './EditPricingStructureModal';
 import { EditingPricingStructure } from './EditingProductTypes';
 import { useTranslation } from 'react-i18next';
 import { AdminPricingStructureDescription } from 'Store/describePricingStructure';
@@ -21,7 +21,13 @@ function PricingStructureInput({ value, onChange }: PricingStructureInputProps):
       <button
         type="button"
         className="btn btn-link py-0"
-        onClick={() => pricingStructureModal.open({ value, onChange, opened: new Date() })}
+        onClick={() =>
+          pricingStructureModal.open({
+            value,
+            onChange: onChange as unknown as PricingStructureModalState['onChange'],
+            opened: new Date(),
+          })
+        }
       >
         <i className="bi-pencil-fill" />
         <span className="visually-hidden">{t('buttons.edit')}</span>
diff --git a/app/javascript/Tables/ColumnSelector.tsx b/app/javascript/Tables/ColumnSelector.tsx
index 5f495d63191..1fe61b26a1a 100644
--- a/app/javascript/Tables/ColumnSelector.tsx
+++ b/app/javascript/Tables/ColumnSelector.tsx
@@ -42,7 +42,7 @@ function ColumnSelector<TData>({
   };
 
   const visibleColumnIds = useMemo(() => {
-    return Object.entries(columnVisibility).reduce((acc, [id, visible]) => {
+    return Object.entries(columnVisibility).reduce((acc: string[], [id, visible]) => {
       if (visible) {
         return [...acc, id];
       } else {
diff --git a/app/javascript/Tables/ReactTableExportButtonWithColumnTransform.tsx b/app/javascript/Tables/ReactTableExportButtonWithColumnTransform.tsx
index 0d318241616..fb755b1aa40 100644
--- a/app/javascript/Tables/ReactTableExportButtonWithColumnTransform.tsx
+++ b/app/javascript/Tables/ReactTableExportButtonWithColumnTransform.tsx
@@ -16,7 +16,7 @@ function ReactTableExportButtonWithColumnTransform({
   ...otherProps
 }: ReactTableExportButtonWithColumnTransformProps): React.JSX.Element {
   const columns = useMemo(
-    () => flatMap(columnVisibilityToVisibleColumnIds(columnVisibility), columnTransform),
+    () => flatMap([...columnVisibilityToVisibleColumnIds(columnVisibility)], columnTransform),
     [columnVisibility, columnTransform],
   );
 
diff --git a/app/javascript/Tables/TableHeader.tsx b/app/javascript/Tables/TableHeader.tsx
index ea0ff4837c7..ac2cb949e18 100644
--- a/app/javascript/Tables/TableHeader.tsx
+++ b/app/javascript/Tables/TableHeader.tsx
@@ -27,7 +27,7 @@ function TableHeader<TData>({
   sortBy,
 }: TableHeaderProps<TData>): React.JSX.Element {
   const visibleColumnIds = useMemo(() => {
-    return Object.entries(columnSelectionProps.columnVisibility).reduce((acc, [id, visible]) => {
+    return Object.entries(columnSelectionProps.columnVisibility).reduce((acc: string[], [id, visible]) => {
       if (visible) {
         return [...acc, id];
       } else {
diff --git a/app/javascript/TicketTypeAdmin/EditTicketProvidingProductModal.tsx b/app/javascript/TicketTypeAdmin/EditTicketProvidingProductModal.tsx
index f98a81f7d47..f2dade3e259 100644
--- a/app/javascript/TicketTypeAdmin/EditTicketProvidingProductModal.tsx
+++ b/app/javascript/TicketTypeAdmin/EditTicketProvidingProductModal.tsx
@@ -1,4 +1,5 @@
 import { useEffect, useState } from 'react';
+import * as React from 'react';
 import { ErrorDisplay, useModal } from '@neinteractiveliterature/litform';
 import { useTranslation } from 'react-i18next';
 import Modal from 'react-bootstrap4-modal';
@@ -61,7 +62,7 @@ export default function EditTicketProvidingProductModal({
                 hideVariants
                 ticketTypes={state ? [state.ticketType] : []}
                 product={product}
-                setProduct={setProduct}
+                setProduct={setProduct as React.Dispatch<React.SetStateAction<EditingProductBase>>}
               />
             )}
           </PricingStructureModalContext.Provider>
diff --git a/app/javascript/TicketTypeAdmin/EditTicketType.tsx b/app/javascript/TicketTypeAdmin/EditTicketType.tsx
index 986b698c08b..eeea5d8837a 100644
--- a/app/javascript/TicketTypeAdmin/EditTicketType.tsx
+++ b/app/javascript/TicketTypeAdmin/EditTicketType.tsx
@@ -1,10 +1,11 @@
 import { useState, useContext } from 'react';
+import * as React from 'react';
 import { ActionFunction, redirect, useFetcher, useLoaderData, RouterContextProvider } from 'react-router';
 
 import { ErrorDisplay } from '@neinteractiveliterature/litform';
 
 import buildTicketTypeInput from './buildTicketTypeInput';
-import TicketTypeForm from './TicketTypeForm';
+import TicketTypeForm, { EditingTicketType } from './TicketTypeForm';
 import usePageTitle from '../usePageTitle';
 import AppRootContext from '../AppRootContext';
 import { SingleTicketTypeLoaderResult } from './loaders';
@@ -49,7 +50,10 @@ function EditTicketTypeForm() {
       <h1 className="mb-4">
         Editing {ticketName} type “{initialTicketType.name}”
       </h1>
-      <TicketTypeForm ticketType={ticketType} onChange={setTicketType} />
+      <TicketTypeForm
+        ticketType={ticketType}
+        onChange={setTicketType as React.Dispatch<React.SetStateAction<EditingTicketType>>}
+      />
       <button type="button" className="btn btn-primary" onClick={saveClicked} disabled={inProgress}>
         Save changes
       </button>
diff --git a/app/javascript/TimeUtils.ts b/app/javascript/TimeUtils.ts
index 04939a2e886..ba0f23ae734 100644
--- a/app/javascript/TimeUtils.ts
+++ b/app/javascript/TimeUtils.ts
@@ -115,19 +115,17 @@ export const timesAreSameOrBothNull = (a?: DateTime | null, b?: DateTime | null)
   return (a == null && b == null) || a?.valueOf() === b?.valueOf();
 };
 
-export const compareTimesAscending = (a: DateTime, b: DateTime): number => {
-  if (a < b) {
-    return -1;
-  }
-
-  if (b < a) {
-    return 1;
-  }
-
+export const compareTimesAscending = (a: DateTime | null | undefined, b: DateTime | null | undefined): number => {
+  if (a == null && b == null) return 0;
+  if (a == null) return -1; // null = -infinity, sorts before real times
+  if (b == null) return 1;
+  if (a < b) return -1;
+  if (b < a) return 1;
   return 0;
 };
 
-export const compareTimesDescending = (a: DateTime, b: DateTime): number => compareTimesAscending(b, a);
+export const compareTimesDescending = (a: DateTime | null | undefined, b: DateTime | null | undefined): number =>
+  compareTimesAscending(b, a);
 
 export function ageAsOf(birthDate?: DateTime | null, date?: DateTime | null): number | null {
   if (!birthDate || !date || !birthDate.isValid || !date.isValid) {
diff --git a/app/javascript/UIComponents/ScheduledValuePreview.tsx b/app/javascript/UIComponents/ScheduledValuePreview.tsx
index 83cdfd68394..9dbac96bba0 100644
--- a/app/javascript/UIComponents/ScheduledValuePreview.tsx
+++ b/app/javascript/UIComponents/ScheduledValuePreview.tsx
@@ -289,8 +289,8 @@ function ScheduledValuePreview<ValueType>({
       <MemoizedScheduledValuePreviewCalendar
         scheduledValue={scheduledValue}
         timezoneName={timezoneName}
-        getClassNameForValue={getClassNameForValue}
-        getDescriptionForValue={getDescriptionForValue}
+        getClassNameForValue={getClassNameForValue as (value: unknown, nextValue: unknown) => string}
+        getDescriptionForValue={getDescriptionForValue as (value: unknown) => ReactNode}
         focusDate={focusDate}
         blurDate={blurDate}
         dateElementMapRef={dateElementMapRef}
diff --git a/app/javascript/UserActivityAlerts/EditUserActivityAlert.tsx b/app/javascript/UserActivityAlerts/EditUserActivityAlert.tsx
index 18c2ab77e9f..290f66a15a7 100644
--- a/app/javascript/UserActivityAlerts/EditUserActivityAlert.tsx
+++ b/app/javascript/UserActivityAlerts/EditUserActivityAlert.tsx
@@ -1,5 +1,14 @@
 import { useState, useMemo } from 'react';
-import { ActionFunction, LoaderFunction, redirect, replace, useFetcher, useLoaderData, RouterContextProvider } from 'react-router';
+import * as React from 'react';
+import {
+  ActionFunction,
+  LoaderFunction,
+  redirect,
+  replace,
+  useFetcher,
+  useLoaderData,
+  RouterContextProvider,
+} from 'react-router';
 
 import { useConfirm, ErrorDisplay } from '@neinteractiveliterature/litform';
 
@@ -138,7 +147,11 @@ function EditUserActivityAlertForm() {
         userActivityAlert={combinedUserActivityAlert}
         convention={convention}
         onChange={setUserActivityAlert}
-        onAddNotificationDestination={addNotificationDestination}
+        onAddNotificationDestination={
+          addNotificationDestination as React.Dispatch<
+            Omit<(typeof userActivityAlert)['notification_destinations'][0], 'id'>
+          >
+        }
         onRemoveNotificationDestination={removeNotificationDestination}
         disabled={inProgress}
         userActivityAlertEvent={userActivityAlertEvent}
diff --git a/app/javascript/UserActivityAlerts/NewUserActivityAlert.tsx b/app/javascript/UserActivityAlerts/NewUserActivityAlert.tsx
index 76795f26898..b7d80dc449a 100644
--- a/app/javascript/UserActivityAlerts/NewUserActivityAlert.tsx
+++ b/app/javascript/UserActivityAlerts/NewUserActivityAlert.tsx
@@ -1,5 +1,13 @@
 import { useState, useMemo } from 'react';
-import { ActionFunction, LoaderFunction, redirect, useFetcher, useLoaderData, RouterContextProvider } from 'react-router';
+import * as React from 'react';
+import {
+  ActionFunction,
+  LoaderFunction,
+  redirect,
+  useFetcher,
+  useLoaderData,
+  RouterContextProvider,
+} from 'react-router';
 
 import { ErrorDisplay } from '@neinteractiveliterature/litform';
 
@@ -128,7 +136,14 @@ function NewUserActivityAlert() {
         userActivityAlert={combinedUserActivityAlert}
         convention={convention}
         onChange={setUserActivityAlert}
-        onAddNotificationDestination={addNotificationDestination}
+        onAddNotificationDestination={
+          addNotificationDestination as React.Dispatch<
+            Omit<
+              UserActivityAlertsAdminQueryData['convention']['user_activity_alerts'][number]['notification_destinations'][number],
+              'id'
+            >
+          >
+        }
         onRemoveNotificationDestination={removeNotificationDestination}
         disabled={createInProgress}
         userActivityAlertEvent={userActivityAlertEvent}
diff --git a/app/javascript/UserConProfiles/AddAttendeeModal.tsx b/app/javascript/UserConProfiles/AddAttendeeModal.tsx
index b15277c1af7..740f87b996c 100644
--- a/app/javascript/UserConProfiles/AddAttendeeModal.tsx
+++ b/app/javascript/UserConProfiles/AddAttendeeModal.tsx
@@ -42,7 +42,8 @@ function AddAttendeeModal(): React.JSX.Element {
     navigate('/user_con_profiles', { replace: true });
   };
 
-  const userSelected = (newUser: UserType) => {
+  const userSelected = (newUser: UserType | null) => {
+    if (!newUser) return;
     setUser(newUser);
     setUserConProfile({
       __typename: 'UserConProfile',
diff --git a/app/javascript/UserConProfiles/ConvertToEventProvidedTicketModal.tsx b/app/javascript/UserConProfiles/ConvertToEventProvidedTicketModal.tsx
index 227ee560031..a8491e10306 100644
--- a/app/javascript/UserConProfiles/ConvertToEventProvidedTicketModal.tsx
+++ b/app/javascript/UserConProfiles/ConvertToEventProvidedTicketModal.tsx
@@ -138,9 +138,11 @@ function ConvertToEventProvidedTicketModal({
 
         <EventSelect
           value={event}
-          onChange={(value: DefaultEventSelectOptionType) => {
-            setEvent(value);
-            setTicketTypeId(undefined);
+          onChange={(value) => {
+            if (value) {
+              setEvent(value);
+              setTicketTypeId(undefined);
+            }
           }}
           isDisabled={inProgress}
         />
diff --git a/app/javascript/UserConProfiles/UserConProfileForm.tsx b/app/javascript/UserConProfiles/UserConProfileForm.tsx
index 425ac6e9937..096030740c3 100644
--- a/app/javascript/UserConProfiles/UserConProfileForm.tsx
+++ b/app/javascript/UserConProfiles/UserConProfileForm.tsx
@@ -22,7 +22,7 @@ function UserConProfileForm<
   UserConProfileType extends FormResponse &
     Pick<UserConProfile, 'current_user_form_item_viewer_role' | 'current_user_form_item_writer_role'>,
 >(props: UserConProfileFormProps<UserConProfileType>): React.JSX.Element {
-  const formResponseValuesChanged = (newResponseValues: UserConProfileType['form_response_attrs']) => {
+  const formResponseValuesChanged = (newResponseValues: Record<string, unknown>) => {
     props.onChange((prevUserConProfile) => ({
       ...prevUserConProfile,
       form_response_attrs: {
diff --git a/app/javascript/UserConProfiles/UserConProfilesTable.tsx b/app/javascript/UserConProfiles/UserConProfilesTable.tsx
index a6ff67a5e8b..fba58d34268 100644
--- a/app/javascript/UserConProfiles/UserConProfilesTable.tsx
+++ b/app/javascript/UserConProfiles/UserConProfilesTable.tsx
@@ -154,38 +154,38 @@ function UserConProfilesTable({
         header: t('admin.userConProfiles.id'),
         id: 'id',
         size: 70,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor('user_id', {
         header: t('admin.userConProfiles.userId'),
         id: 'user_id',
         size: 70,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor((userConProfile) => userConProfile, {
         header: t('admin.userConProfiles.name'),
         id: 'name',
         enableColumnFilter: true,
         enableSorting: true,
         cell: UserConProfileWithGravatarCell,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor('first_name', {
         header: t('admin.userConProfiles.firstName'),
         id: 'first_name',
         enableColumnFilter: true,
         enableSorting: true,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor('last_name', {
         header: t('admin.userConProfiles.lastName'),
         id: 'last_name',
         enableColumnFilter: true,
         enableSorting: true,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor('email', {
         header: t('admin.userConProfiles.email'),
         id: 'email',
         enableColumnFilter: true,
         enableSorting: true,
         cell: EmailCell,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
     ];
 
     if (attendeesPageQueryData.convention.ticket_mode !== TicketMode.Disabled && canReadTickets) {
@@ -197,7 +197,7 @@ function UserConProfilesTable({
           enableColumnFilter: true,
           enableSorting: true,
           cell: TicketStatusWithPaymentAmountCell,
-        }),
+        }) as ColumnDef<UserConProfilesTableRow>,
         columnHelper.accessor('ticket', {
           header: t('admin.userConProfiles.ticketType', {
             ticketName: humanize(attendeesPageQueryData.convention.ticket_name || 'ticket'),
@@ -207,7 +207,7 @@ function UserConProfilesTable({
           enableColumnFilter: true,
           enableSorting: true,
           cell: TicketStatusCell,
-        }),
+        }) as ColumnDef<UserConProfilesTableRow>,
         columnHelper.accessor('ticket', {
           header: t('admin.userConProfiles.paymentAmount'),
           id: 'payment_amount',
@@ -215,7 +215,7 @@ function UserConProfilesTable({
           enableColumnFilter: true,
           enableSorting: true,
           cell: TicketPaymentAmountCell,
-        }),
+        }) as ColumnDef<UserConProfilesTableRow>,
       );
     }
 
@@ -226,7 +226,7 @@ function UserConProfilesTable({
         size: 150,
         enableColumnFilter: true,
         cell: BooleanCell,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
     );
 
     if (canReadTickets) {
@@ -237,7 +237,7 @@ function UserConProfilesTable({
           size: 150,
           enableColumnFilter: true,
           cell: BooleanCell,
-        }),
+        }) as ColumnDef<UserConProfilesTableRow>,
       );
     }
 
@@ -254,7 +254,7 @@ function UserConProfilesTable({
             enableSorting: true,
             cell: TicketStatusChangeCell,
           },
-        ),
+        ) as ColumnDef<UserConProfilesTableRow>,
       );
     }
 
@@ -265,11 +265,11 @@ function UserConProfilesTable({
         enableColumnFilter: true,
         enableSorting: true,
         cell: PrivilegesCell,
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
       columnHelper.accessor('order_summary', {
         header: t('admin.userConProfiles.orderSummary'),
         id: 'order_summary',
-      }),
+      }) as ColumnDef<UserConProfilesTableRow>,
     );
 
     const existingColumnIds = new Set(columns.map((column) => column.id));
@@ -298,7 +298,7 @@ function UserConProfilesTable({
             id: identifier,
             cell: FormItemCell,
           },
-        ),
+        ) as ColumnDef<UserConProfilesTableRow>,
       );
     });
 
diff --git a/app/javascript/packs/application.tsx b/app/javascript/packs/application.tsx
index e4523762d17..fb67828f116 100644
--- a/app/javascript/packs/application.tsx
+++ b/app/javascript/packs/application.tsx
@@ -1,6 +1,7 @@
 import 'regenerator-runtime/runtime';
 
 import mountReactComponents from '../mountReactComponents';
+import * as React from 'react';
 import { StrictMode, use, useEffect, useMemo, useState } from 'react';
 import AuthenticityTokensManager, { getAuthenticityTokensURL } from 'AuthenticityTokensContext';
 import { createBrowserRouter, RouterContextProvider, RouterProvider } from 'react-router';
@@ -91,4 +92,4 @@ function DataModeApplicationEntry({
   return <StrictMode>{router && <RouterProvider router={router} />}</StrictMode>;
 }
 
-mountReactComponents({ AppRoot: DataModeApplicationEntry });
+mountReactComponents({ AppRoot: DataModeApplicationEntry as React.ComponentType<unknown> });
diff --git a/app/javascript/parsePageContent.tsx b/app/javascript/parsePageContent.tsx
index 495cc1b126e..a6005720f94 100644
--- a/app/javascript/parsePageContent.tsx
+++ b/app/javascript/parsePageContent.tsx
@@ -138,7 +138,7 @@ function jsxAttributeKeyForHtmlKey(htmlKey: string) {
 function jsxAttributesFromHTMLAttributes(node: Element, attributes: Attr[]) {
   const testingNode = document.createElement(node.tagName);
 
-  return attributes.reduce((result: { [key: string]: string }, attribute: Attr) => {
+  return attributes.reduce<{ [key: string]: string | (() => void) | CSSProperties }>((result, attribute) => {
     const key = jsxAttributeKeyForHtmlKey(attribute.name);
     if (key === 'style') {
       return { ...result, [key]: createStyleJsonFromString(attribute.value) };
@@ -259,7 +259,11 @@ function processCmsLinkNode(node: Element, children: ReactNode[], index: number)
 
   if (href && !href.startsWith('#') && getURLOrigin(href) === window.location.origin) {
     return (
-      <Link to={href} key={index} {...jsxAttributesFromHTMLAttributes(node, otherAttributes)}>
+      <Link
+        to={href}
+        key={index}
+        {...(jsxAttributesFromHTMLAttributes(node, otherAttributes) as React.AnchorHTMLAttributes<HTMLAnchorElement>)}
+      >
         {children}
       </Link>
     );
@@ -306,21 +310,28 @@ function traverseWithInstructions(
   return list.length <= 1 ? list[0] : list;
 }
 
-function buildProcessingInstructions(componentMap: ComponentMap): ProcessingInstruction<unknown>[] {
+type AnyProcessingInstruction = ProcessingInstruction<unknown>;
+
+function buildProcessingInstructions(componentMap: ComponentMap): AnyProcessingInstruction[] {
   return [
-    ...AUTHENTICATION_LINK_PROCESSING_INSTRUCTIONS,
+    ...(AUTHENTICATION_LINK_PROCESSING_INSTRUCTIONS as AnyProcessingInstruction[]),
     {
       shouldProcessNode: (node) => nodeIsElement(node) && node.nodeName.toLowerCase() === 'a',
-      processNode: processCmsLinkNode,
+      processNode: processCmsLinkNode as unknown as AnyProcessingInstruction['processNode'],
     },
     {
       shouldProcessNode: (node) => nodeIsElement(node) && node.attributes.getNamedItem('data-react-class') != null,
-      processNode: (node: Element, children: ReactNode[], index: number) =>
-        processReactComponentNode(node, children, index, componentMap),
+      processNode: ((node: Element, children: ReactNode[], index: number) =>
+        processReactComponentNode(
+          node,
+          children,
+          index,
+          componentMap,
+        )) as unknown as AnyProcessingInstruction['processNode'],
     },
     {
       shouldProcessNode: () => true,
-      processNode: processDefaultNode,
+      processNode: processDefaultNode as unknown as AnyProcessingInstruction['processNode'],
     },
   ];
 }
diff --git a/app/javascript/parseResponseErrors.tsx b/app/javascript/parseResponseErrors.tsx
index 4d886ed6cb2..e6363ad1d4a 100644
--- a/app/javascript/parseResponseErrors.tsx
+++ b/app/javascript/parseResponseErrors.tsx
@@ -10,7 +10,7 @@ export class FormValidationError extends Error {
   }
 }
 
-export function parseResponseErrors(error: Error, errorPath: readonly (string | number)[]) {
+export function parseResponseErrors(error: unknown, errorPath: readonly (string | number)[]) {
   if (CombinedGraphQLErrors.is(error)) {
     const updateError = error.errors.find((graphQLError) => isEqual(graphQLError.path, errorPath));
     const validationErrors = (updateError?.extensions?.validationErrors as Record<string, string[]>) ?? {};
diff --git a/app/javascript/setupI18Next.ts b/app/javascript/setupI18Next.ts
index ee40496ef75..35e5245495b 100644
--- a/app/javascript/setupI18Next.ts
+++ b/app/javascript/setupI18Next.ts
@@ -80,7 +80,7 @@ const i18nInitPromise = i18nObject.init(initOptions).then(() => {
   );
   i18nObject.services.formatter?.add(
     'datetimenamed',
-    (value: DateTime | string | undefined | null, lng: string, options: { format: DateTimeFormatKey }) => {
+    (value: DateTime | string | undefined | null, lng: string | undefined, options: { format: DateTimeFormatKey }) => {
       let dateTimeValue: DateTime;
       if (value == null) {
         return '';
diff --git a/app/javascript/useAsyncFetcher.ts b/app/javascript/useAsyncFetcher.ts
index 789cfeb2a33..0e99936334d 100644
--- a/app/javascript/useAsyncFetcher.ts
+++ b/app/javascript/useAsyncFetcher.ts
@@ -11,7 +11,7 @@ export function useAsyncFetcher() {
         if (resolveRef.current) {
           reject(new Error('Request already in progress'));
         } else {
-          resolveRef.current = resolve;
+          resolveRef.current = resolve as (data: unknown) => void;
           fetcher.load(...args);
         }
       }),
@@ -24,7 +24,7 @@ export function useAsyncFetcher() {
         if (resolveRef.current) {
           reject(new Error('Request already in progress'));
         } else {
-          resolveRef.current = resolve;
+          resolveRef.current = resolve as (data: unknown) => void;
           fetcher.submit(...args);
         }
       }),
diff --git a/app/javascript/useAsyncFunction.ts b/app/javascript/useAsyncFunction.ts
index 4116c991e11..1f27eaf4795 100644
--- a/app/javascript/useAsyncFunction.ts
+++ b/app/javascript/useAsyncFunction.ts
@@ -31,7 +31,7 @@ export default function useAsyncFunction<T, A extends unknown[]>(
           return await func(...args);
         } catch (e) {
           if (isMounted.current) {
-            setError(e);
+            setError(e as Error);
           }
           if (!suppressError) {
             throw e;
diff --git a/package.json b/package.json
index 90b69341819..f7d41bf1b73 100644
--- a/package.json
+++ b/package.json
@@ -186,9 +186,9 @@
     "@types/qs": "^6",
     "@types/rails__activestorage": "8.0.1",
     "@types/react-dom": "19.2.3",
-    "@typescript-eslint/eslint-plugin": "8.46.4",
-    "@typescript-eslint/parser": "8.46.4",
-    "@typescript-eslint/utils": "^8.36.0",
+    "@typescript-eslint/eslint-plugin": "8.58.0",
+    "@typescript-eslint/parser": "8.58.0",
+    "@typescript-eslint/utils": "^8.58.0",
     "@vitejs/plugin-react-swc": "^4.2.0",
     "@vitest/coverage-v8": "^4.0.0",
     "@vitest/eslint-plugin": "^1.1.24",
@@ -223,8 +223,8 @@
     "tslib": "^2.8.1",
     "tsx": "^4.20.3",
     "typed-scss-modules": "^8.1.1",
-    "typescript": "5.9.3",
-    "typescript-eslint": "^8.36.0",
+    "typescript": "6.0.2",
+    "typescript-eslint": "^8.58.0",
     "vite": "^7.0.0",
     "vite-bundle-visualizer": "^1.2.1",
     "vite-plugin-node-polyfills": "^0.24.0",
diff --git a/test/javascript/BuiltInFormControls/ConventionDaySelect.test.tsx b/test/javascript/BuiltInFormControls/ConventionDaySelect.test.tsx
index a98e0037405..1b29c0c04b1 100644
--- a/test/javascript/BuiltInFormControls/ConventionDaySelect.test.tsx
+++ b/test/javascript/BuiltInFormControls/ConventionDaySelect.test.tsx
@@ -30,7 +30,7 @@ describe('ConventionDaySelect', () => {
 
   test('it renders an option for each convention day', async () => {
     const { getAllByRole } = await renderConventionDaySelect();
-    expect(getAllByRole('radio').map((input: HTMLInputElement) => input.value)).toEqual([
+    expect(getAllByRole('radio').map((input: HTMLElement) => (input as HTMLInputElement).value)).toEqual([
       '2017-01-01T00:00:00.000+00:00',
       '2017-01-02T06:00:00.000+00:00',
       '2017-01-03T06:00:00.000+00:00',
diff --git a/test/javascript/BuiltInFormControls/GraphQLAsyncSelect.test.tsx b/test/javascript/BuiltInFormControls/GraphQLAsyncSelect.test.tsx
index d7395378226..e6aba99e8eb 100644
--- a/test/javascript/BuiltInFormControls/GraphQLAsyncSelect.test.tsx
+++ b/test/javascript/BuiltInFormControls/GraphQLAsyncSelect.test.tsx
@@ -5,7 +5,7 @@ import { render, fireEvent, waitFor } from '../testUtils';
 import GraphQLAsyncSelect, {
   GraphQLAsyncSelectProps,
 } from '../../../app/javascript/BuiltInFormControls/GraphQLAsyncSelect';
-import { ResultOf } from '@graphql-typed-document-node/core';
+import { ResultOf, VariablesOf } from '@graphql-typed-document-node/core';
 import { FakeQueryData, FakeQueryDocument, FakeQueryVariables } from './graphQLAsyncSelect.queries.generated';
 
 const defaultQueryMock: MockedResponse<FakeQueryData, FakeQueryVariables> = {
@@ -48,11 +48,11 @@ describe('GraphQLAsyncSelect', () => {
   ) =>
     render(
       <GraphQLAsyncSelect
-        query={FakeQueryDocument}
+        query={FakeQueryDocument as QueryType}
         getOptions={(data) => data.convention.user_con_profiles_paginated.entries as OptionType[]}
         getOptionLabel={(option) => option.name_without_nickname}
         getOptionValue={(option) => option.id}
-        getVariables={(input) => ({ name: input })}
+        getVariables={(input) => ({ name: input }) as VariablesOf<QueryType>}
         {...props}
       />,
       { apolloMocks: mocks ?? defaultMocks },
diff --git a/test/javascript/BuiltInFormControls/buildTestScheduledValueInput.tsx b/test/javascript/BuiltInFormControls/buildTestScheduledValueInput.tsx
index 67a4cbf5c78..eea538724a5 100644
--- a/test/javascript/BuiltInFormControls/buildTestScheduledValueInput.tsx
+++ b/test/javascript/BuiltInFormControls/buildTestScheduledValueInput.tsx
@@ -1,7 +1,7 @@
 import * as React from 'react';
 
 export default function buildTestScheduledValueInput(
-  value: string,
+  value: string | number | undefined,
   onChange: React.Dispatch<string>,
 ): React.JSX.Element {
   const processChangeEvent = (event: React.ChangeEvent<HTMLInputElement>) => {
diff --git a/test/javascript/RegistrationPolicy/RegistrationPolicyEditor.test.tsx b/test/javascript/RegistrationPolicy/RegistrationPolicyEditor.test.tsx
index 352fb1eca82..c55ee5a2854 100644
--- a/test/javascript/RegistrationPolicy/RegistrationPolicyEditor.test.tsx
+++ b/test/javascript/RegistrationPolicy/RegistrationPolicyEditor.test.tsx
@@ -4,12 +4,12 @@ import RegistrationPolicyEditor, {
   EditingRegistrationPolicy,
   RegistrationPolicyEditorProps,
 } from '../../../app/javascript/RegistrationPolicy/RegistrationPolicyEditor';
-import { RegistrationPolicy, RegistrationPolicyBucket } from '../../../app/javascript/graphqlTypes.generated';
+import { RegistrationPolicyBucket } from '../../../app/javascript/graphqlTypes.generated';
 import { EditingRegistrationBucket } from '../../../app/javascript/RegistrationPolicy/RegistrationBucketRow';
 import { vi } from 'vitest';
 
 describe('RegistrationPolicyEditor', () => {
-  const onChange = vi.fn<(rp: RegistrationPolicy) => void>();
+  const onChange = vi.fn<(rp: EditingRegistrationPolicy<EditingRegistrationBucket>) => void>();
   beforeEach(onChange.mockReset);
 
   const defaultRegistrationPolicyBucket: RegistrationPolicyBucket = {
diff --git a/tsconfig.json b/tsconfig.json
index ec08f608165..c7ad00b8f02 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -14,10 +14,12 @@
     "noImplicitAny": true,
     "skipLibCheck": true,
     "maxNodeModuleJsDepth": 10,
-    "baseUrl": "./app/javascript",
     "types": ["vitest/globals", "vite/client"],
     "incremental": true,
-    "tsBuildInfoFile": ".tsbuildinfo"
+    "tsBuildInfoFile": ".tsbuildinfo",
+    "paths": {
+      "*": ["./app/javascript/*"]
+    }
   },
   "include": [
     "./app/javascript/",
diff --git a/yarn.lock b/yarn.lock
index cb5188c9577..5f39c1260f5 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4150,13 +4150,31 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@eslint-community/regexpp@npm:^4.10.0, @eslint-community/regexpp@npm:^4.12.1":
+"@eslint-community/eslint-utils@npm:^4.9.1":
+  version: 4.9.1
+  resolution: "@eslint-community/eslint-utils@npm:4.9.1"
+  dependencies:
+    eslint-visitor-keys: "npm:^3.4.3"
+  peerDependencies:
+    eslint: ^6.0.0 || ^7.0.0 || >=8.0.0
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-    "@typescript-eslint/visitor-keys": "npm:8.46.4"
-    graphemer: "npm:^1.4.0"
-    ignore: "npm:^7.0.0"
+    "@eslint-community/regexpp": "npm:^4.12.2"
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+    "@typescript-eslint/type-utils": "npm:8.58.0"
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+    "@typescript-eslint/visitor-keys": "npm:8.58.0"
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-    ts-api-utils: "npm:^2.1.0"
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+  resolution: "@typescript-eslint/project-service@npm:8.58.0"
+  dependencies:
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+    "@typescript-eslint/types": "npm:^8.58.0"
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+  resolution: "@typescript-eslint/scope-manager@npm:8.58.0"
+  dependencies:
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+  languageName: node
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+  resolution: "@typescript-eslint/tsconfig-utils@npm:8.58.0"
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-    "@typescript-eslint/utils": "npm:8.46.4"
-    debug: "npm:^4.3.4"
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+    "@typescript-eslint/typescript-estree": "npm:8.58.0"
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+    debug: "npm:^4.4.3"
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+    "@typescript-eslint/visitor-keys": "npm:8.58.0"
+    debug: "npm:^4.4.3"
+    minimatch: "npm:^10.2.2"
+    semver: "npm:^7.7.3"
+    tinyglobby: "npm:^0.2.15"
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+    "@typescript-eslint/typescript-estree": "npm:8.58.0"
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+  resolution: "@typescript-eslint/visitor-keys@npm:8.58.0"
+  dependencies:
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+  languageName: node
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+  resolution: "brace-expansion@npm:5.0.5"
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+  resolution: "eslint-visitor-keys@npm:5.0.1"
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-    "@typescript-eslint/utils": "npm:^8.36.0"
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+    "@typescript-eslint/parser": "npm:8.58.0"
+    "@typescript-eslint/utils": "npm:^8.58.0"
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-    typescript: "npm:5.9.3"
-    typescript-eslint: "npm:^8.36.0"
+    typescript: "npm:6.0.2"
+    typescript-eslint: "npm:^8.58.0"
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   resolution: "ts-invariant@npm:0.3.3"
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-  resolution: "typescript-eslint@npm:8.46.4"
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