Upgrade to typescript-eslint v6 (#25904)

app/javascript/mastodon/polyfills/base_polyfills.ts

@@ -4,7 +4,7 @@ import 'core-js/features/symbol';
 import 'core-js/features/promise/finally';
 import { decode as decodeBase64 } from '../utils/base64';
 
-if (!HTMLCanvasElement.prototype.toBlob) {
+if (!Object.hasOwn(HTMLCanvasElement.prototype, 'toBlob')) {
   const BASE64_MARKER = ';base64,';
 
   Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
@@ -17,7 +17,7 @@ if (!HTMLCanvasElement.prototype.toBlob) {
       const dataURL: string = this.toDataURL(type, quality);
       let data;
 
-      if (dataURL.indexOf(BASE64_MARKER) >= 0) {
+      if (dataURL.includes(BASE64_MARKER)) {
         const [, base64] = dataURL.split(BASE64_MARKER);
         data = decodeBase64(base64);
       } else {

app/javascript/mastodon/polyfills/index.ts

@@ -24,6 +24,7 @@ export function loadPolyfills() {
   // Latest version of Firefox and Safari do not have IntersectionObserver.
   // Edge does not have requestIdleCallback.
   // This avoids shipping them all the polyfills.
+  /* eslint-disable @typescript-eslint/no-unnecessary-condition -- those properties might not exist in old browsers, even if they are always here in types */
   const needsExtraPolyfills = !(
     window.AbortController &&
     window.IntersectionObserver &&
@@ -31,6 +32,7 @@ export function loadPolyfills() {
     'isIntersecting' in IntersectionObserverEntry.prototype &&
     window.requestIdleCallback
   );
+  /* eslint-enable @typescript-eslint/no-unnecessary-condition */
 
   return Promise.all([
     loadIntlPolyfills(),

app/javascript/mastodon/polyfills/intl.ts

@@ -80,6 +80,7 @@ async function loadIntlPluralRulesPolyfills(locale: string) {
 // }
 
 export async function loadIntlPolyfills() {
+  // eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing -- we want to match empty strings
   const locale = document.querySelector('html')?.lang || 'en';
 
   // order is important here