diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
index 2e08cd52bc..161327e01c 100644
--- a/.github/workflows/validate.yml
+++ b/.github/workflows/validate.yml
@@ -63,7 +63,7 @@ jobs:
             sed -e 's/^\[warn] \(.*\)$/::warning file=\1,line=1::File was not formatted with prettier. Usually, commenting !format will fix this./' \
           ))
 
-  type-warnings:
+  typecheck:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -77,8 +77,8 @@ jobs:
           cache: npm
       - name: Install dependencies
         run: npm ci
-      - name: Generate type warnings
-        run: node development/ci-generate-type-warnings.js
+      - name: Check types
+        run: node development/ci-type-check.js
         env:
           PR_NUMBER: "${{ github.event.number }}"
           GH_REPO: "${{ github.repository }}"
diff --git a/development/builder.js b/development/builder.js
index 181e8a8a7d..7d4894d034 100644
--- a/development/builder.js
+++ b/development/builder.js
@@ -12,6 +12,7 @@ import parseMetadata from "./parse-extension-metadata.js";
 import parseTranslations from "./parse-extension-translations.js";
 import renderTemplate from "./render-template.js";
 import renderDocs from "./render-docs.js";
+import transpileTypeScript from "./transpile-typescript.js";
 import { mkdirp, recursiveReadDirectory } from "./fs-utils.js";
 import {
   fetchAllDependencies,
@@ -176,8 +177,27 @@ class ExtensionFile extends BuildFile {
     this.mode = mode;
   }
 
+  getType() {
+    return ".js";
+  }
+
+  /**
+   * @returns {{jsCode: string; sourceMap: string | null}}
+   */
+  async transpile() {
+    const sourceMap = this.mode === "development";
+    const source = await fsPromises.readFile(this.sourcePath, "utf-8");
+    if (this.sourcePath.endsWith(".ts")) {
+      return transpileTypeScript(source, this.slug, !!sourceMap);
+    }
+    return {
+      jsCode: source,
+      sourceMap: null,
+    };
+  }
+
   async read() {
-    let data = await fsPromises.readFile(this.sourcePath, "utf-8");
+    let data = (await this.transpile()).jsCode;
 
     if (this.mode !== "development") {
       const dependenciesJS = rewriteExternalToInline(data);
@@ -186,6 +206,11 @@ class ExtensionFile extends BuildFile {
       let prefixJS = "";
       let suffixJS = "";
 
+      if (this.sourcePath.endsWith(".ts")) {
+        prefixJS +=
+          "/* transpiled from TypeScript - see repository for original version with types */";
+      }
+
       const translations = filterTranslationsByPrefix(
         this.allTranslations,
         `${this.slug}@`
@@ -286,10 +311,10 @@ class ExtensionFile extends BuildFile {
       }
     }
 
-    validateImports(js);
+    validateImports((await this.transpile()).jsCode);
   }
 
-  getStrings() {
+  async getStrings() {
     if (!this.featured) {
       return null;
     }
@@ -315,7 +340,7 @@ class ExtensionFile extends BuildFile {
       },
     };
 
-    const jsCode = fs.readFileSync(this.sourcePath, "utf-8");
+    const jsCode = (await this.transpile()).jsCode;
     const unprefixedRuntimeStrings = parseTranslations(jsCode);
     const runtimeStrings = Object.fromEntries(
       Object.entries(unprefixedRuntimeStrings).map(([key, value]) => [
@@ -331,9 +356,26 @@ class ExtensionFile extends BuildFile {
   }
 
   async getDependencies() {
-    return parseExtensionDependencies(
-      await fsPromises.readFile(this.sourcePath, "utf-8")
-    );
+    return parseExtensionDependencies((await this.transpile()).jsCode);
+  }
+}
+
+class ExtensionSourceMapFile extends BuildFile {
+  /**
+   * @param {ExtensionFile} extensionFile
+   */
+  constructor(extensionFile) {
+    super(extensionFile.sourcePath);
+    /** @type {ExtensionFile} */
+    this.extensionFile = extensionFile;
+  }
+
+  getType() {
+    return ".json";
+  }
+
+  async read() {
+    return (await this.extensionFile.transpile()).sourceMap;
   }
 }
 
@@ -675,13 +717,13 @@ class Build {
   /**
    * @returns {Record<string, Record<string, TranslatableString>>}
    */
-  generateL10N() {
+  async generateL10N() {
     const allStrings = {};
 
     for (const [filePath, file] of Object.entries(this.files)) {
       let fileStrings;
       try {
-        fileStrings = file.getStrings();
+        fileStrings = await file.getStrings();
       } catch (error) {
         console.error(error);
         throw new Error(
@@ -717,7 +759,7 @@ class Build {
   async exportL10N(root) {
     await mkdirp(root);
 
-    const groups = this.generateL10N();
+    const groups = await this.generateL10N();
     for (const [name, strings] of Object.entries(groups)) {
       const filename = pathUtil.join(root, `exported-${name}.json`);
       await fsPromises.writeFile(filename, JSON.stringify(strings, null, 2));
@@ -799,20 +841,34 @@ class Builder {
     for (const [filename, absolutePath] of await recursiveReadDirectory(
       this.extensionsRoot
     )) {
-      if (!filename.endsWith(".js")) {
+      const isJavaScript = filename.endsWith(".js");
+      const isTypeScript =
+        filename.endsWith(".ts") && !filename.endsWith(".d.ts");
+      if (!isJavaScript && !isTypeScript) {
+        // Not an extension.
         continue;
       }
+
       const extensionSlug = filename.split(".")[0];
       const featured = featuredExtensionSlugs.includes(extensionSlug);
-      const file = new ExtensionFile(
+
+      const extensionFile = new ExtensionFile(
         absolutePath,
         extensionSlug,
         featured,
         translations["extension-runtime"],
         this.mode
       );
-      extensionFiles[extensionSlug] = file;
-      build.files[`/${filename}`] = file;
+      extensionFiles[extensionSlug] = extensionFile;
+      build.files[`/${extensionSlug}.js`] = extensionFile;
+
+      // Sourcemaps are only accurate in development builds as production builds will insert strings
+      // and cached dependencies.
+      if (isTypeScript && this.mode === "development") {
+        build.files[`/${extensionSlug}.js.map`] = new ExtensionSourceMapFile(
+          extensionFile
+        );
+      }
     }
 
     /** @type {Record<string, ImageFile>} */
diff --git a/development/ci-generate-type-warnings.js b/development/ci-type-check.js
similarity index 67%
rename from development/ci-generate-type-warnings.js
rename to development/ci-type-check.js
index 26652165a5..ec85908f07 100644
--- a/development/ci-generate-type-warnings.js
+++ b/development/ci-type-check.js
@@ -3,9 +3,8 @@ import ts from "typescript";
 import { createAnnotation, getChangedFiles, isCI } from "./ci-interop.js";
 
 /**
- * @fileoverview Generates CI annotations for type warnings in files modified by the
- * pull request. Standard TypeScript CLI will include all files and its output is
- * detected as errors, so not usable for us.
+ * @fileoverview Generates CI annotations for type checking.
+ * JavaScript files are warn-only; TypeScript are hard errors.
  */
 
 const check = async () => {
@@ -16,7 +15,7 @@ const check = async () => {
     pathUtil.join(rootDir, f)
   );
 
-  console.log(`${changedFiles.size} changed files:`);
+  console.log(`${changedFiles.length} changed files:`);
   console.log(Array.from(changedFiles).sort().join("\n"));
   console.log("");
 
@@ -39,12 +38,25 @@ const check = async () => {
   ];
 
   let numWarnings = 0;
+  let numErrors = 0;
 
   for (const diagnostic of diagnostics) {
-    if (!changedFilesAbsolute.includes(diagnostic.file.fileName)) {
+    const isBlocker = diagnostic.file.fileName.endsWith(".ts");
+
+    if (
+      !isBlocker &&
+      !changedFilesAbsolute.includes(diagnostic.file.fileName)
+    ) {
+      // Warning in a file not touched by the PR: ignore
       continue;
     }
 
+    if (isBlocker) {
+      numErrors++;
+    } else {
+      numWarnings++;
+    }
+
     const startPosition = ts.getLineAndCharacterOfPosition(
       diagnostic.file,
       diagnostic.start
@@ -60,12 +72,12 @@ const check = async () => {
       0
     );
 
-    numWarnings++;
     createAnnotation({
-      type: "warning",
+      type: isBlocker ? "error" : "warning",
       file: diagnostic.file.fileName,
-      title: "Type warning - may indicate a bug - ignore if no bug",
-      onlyIfChanged: true,
+      title: isBlocker
+        ? "Type error - must be fixed"
+        : "Type warning - may indicate a bug - ignore if no bug",
       message: flattened,
       line: startPosition.line + 1,
       col: startPosition.character + 1,
@@ -74,12 +86,17 @@ const check = async () => {
     });
   }
 
+  console.log(`Errors: ${numErrors}`);
   console.log(`Warnings in changed files: ${numWarnings}`);
-  console.log(`Warnings in all files: ${diagnostics.length}`);
+  console.log(`Total errors+warnings in all files: ${diagnostics.length}`);
+
+  return numErrors === 0;
 };
 
 if (isCI()) {
-  check();
+  check().then((success) => {
+    process.exit(success ? 0 : 1);
+  });
 } else {
   console.error(
     "This script is only intended to be used in CI. For development, use normal TypeScript CLI instead."
diff --git a/development/transpile-typescript.js b/development/transpile-typescript.js
new file mode 100644
index 0000000000..ffb17fed84
--- /dev/null
+++ b/development/transpile-typescript.js
@@ -0,0 +1,60 @@
+import * as pathUtil from "node:path";
+import ts from "typescript";
+
+/**
+ * @fileoverview Transpiles TypeScript to JavaScript.
+ */
+
+const tsconfigPath = pathUtil.join(import.meta.dirname, "../tsconfig.json");
+
+/**
+ * @type {import("typescript").CompilerOptions | null}
+ */
+let cachedBaseOptions = null;
+
+/**
+ * @returns {import("typescript").CompilerOptions}
+ */
+const getBaseCompilerOptions = () => {
+  if (!cachedBaseOptions) {
+    const configFile = ts.readConfigFile(tsconfigPath, ts.sys.readFile);
+    if (configFile.error) {
+      throw new Error(
+        ts.flattenDiagnosticMessageText(configFile.error.messageText, "\n")
+      );
+    }
+    const parsed = ts.parseJsonConfigFileContent(
+      configFile.config,
+      ts.sys,
+      pathUtil.dirname(tsconfigPath)
+    );
+    cachedBaseOptions = parsed.options;
+  }
+  return cachedBaseOptions;
+};
+
+/**
+ * Transpile a TypeScript extension to JavaScript. Does not enforce any sort of type checking.
+ * @param {string} tsCode TypeScript code
+ * @param {string} slug extension slug
+ * @param {boolean} sourceMap true to generate source map
+ * @returns {{jsCode: string, sourcemap: string|null}}
+ */
+const compileTypeScript = (tsCode, slug, sourceMap) => {
+  const result = ts.transpileModule(tsCode, {
+    // used in the generated source map
+    fileName: `${slug}.ts`,
+    compilerOptions: {
+      ...getBaseCompilerOptions(),
+      // Our normal `tsc` is checking only - but here we want output
+      noEmit: false,
+      sourceMap,
+    },
+  });
+  return {
+    jsCode: result.outputText,
+    sourceMap: result.sourceMapText ?? null,
+  };
+};
+
+export default compileTypeScript;
diff --git a/development/upload-translations.js b/development/upload-translations.js
index 34266a3647..7be0072967 100644
--- a/development/upload-translations.js
+++ b/development/upload-translations.js
@@ -55,7 +55,7 @@ const run = async () => {
   const build = await builder.build();
 
   console.log("Generating strings...");
-  const l10n = build.generateL10N();
+  const l10n = await build.generateL10N();
 
   console.log("Uploading runtime strings...");
   await uploadRuntimeStrings(l10n["extension-runtime"]);
diff --git a/extensions/cursor.js b/extensions/cursor.ts
similarity index 92%
rename from extensions/cursor.js
rename to extensions/cursor.ts
index 55c927b173..e8304287ec 100644
--- a/extensions/cursor.js
+++ b/extensions/cursor.ts
@@ -5,7 +5,7 @@
 // By: GarboMuffin
 // License: MIT AND MPL-2.0
 
-(function (Scratch) {
+(function (Scratch: typeof globalThis.Scratch) {
   "use strict";
 
   if (!Scratch.extensions.unsandboxed) {
@@ -13,17 +13,13 @@
   }
 
   const lazilyCreatedCanvas = () => {
-    /** @type {HTMLCanvasElement} */
-    let canvas = null;
-    /** @type {CanvasRenderingContext2D} */
-    let ctx = null;
-    /**
-     * @param {number} width
-     * @param {number} height
-     * @returns {[HTMLCanvasElement, CanvasRenderingContext2D]}
-     */
-    return (width, height) => {
-      if (!canvas) {
+    let canvas: HTMLCanvasElement | null = null;
+    let ctx: CanvasRenderingContext2D | null = null;
+    return (
+      width: number,
+      height: number
+    ): [HTMLCanvasElement, CanvasRenderingContext2D] => {
+      if (!canvas || !ctx) {
         canvas = document.createElement("canvas");
         ctx = canvas.getContext("2d");
         if (!ctx) {
@@ -38,15 +34,17 @@
   };
   const getRawSkinCanvas = lazilyCreatedCanvas();
 
+  const isSVGSkin = (skin: RenderWebGL.Skin): skin is RenderWebGL.SVGSkin =>
+    !!(skin as RenderWebGL.SVGSkin)._svgImage;
+
   /**
-   * @param {RenderWebGL.Skin} skin
-   * @returns {string} A data: URI for the skin.
+   * @param skin
+   * @returns A data: URI for the skin.
    */
-  const encodeSkinToURL = (skin) => {
-    const svgSkin = /** @type {RenderWebGL.SVGSkin} */ (skin);
-    if (svgSkin._svgImage) {
+  const encodeSkinToURL = (skin: RenderWebGL.Skin): string => {
+    if (isSVGSkin(skin)) {
       // This is an SVG skin
-      return svgSkin._svgImage.src;
+      return skin._svgImage.src;
     }
 
     // It's probably a bitmap skin.
@@ -57,7 +55,9 @@
     if (silhouette.unlazy) {
       silhouette.unlazy();
     }
-    const colorData = silhouette._colorData;
+    // The renderer types _colorData with a generic ArrayBufferLike backing, but the
+    // ImageData constructor specifically wants an ArrayBuffer-backed array.
+    const colorData = silhouette._colorData as Uint8ClampedArray<ArrayBuffer>;
     const width = silhouette._width;
     const height = silhouette._height;
     const imageData = new ImageData(
@@ -70,13 +70,11 @@
     return canvas.toDataURL();
   };
 
-  /**
-   * @param {VM.Costume} costume
-   * @param {number} maxWidth
-   * @param {number} maxHeight
-   * @returns {{uri: string, width: number, height: number}}
-   */
-  const costumeToCursor = (costume, maxWidth, maxHeight) => {
+  const costumeToCursor = (
+    costume: VM.Costume,
+    maxWidth: number,
+    maxHeight: number
+  ): { uri: string; width: number; height: number } => {
     const skin = Scratch.vm.renderer._allSkins[costume.skinId];
     const imageURI = encodeSkinToURL(skin);
 
@@ -113,13 +111,10 @@
     };
   };
 
-  /** @type {string} */
   let nativeCursor = "default";
-  /** @type {null|string} */
-  let customCursorImageName = null;
+  let customCursorImageName: null | string = null;
 
   const canvas = Scratch.renderer.canvas;
-  /** @type {string} */
   let currentCanvasCursor = nativeCursor;
   const updateCanvasCursor = () => {
     if (canvas.style.cursor !== currentCanvasCursor) {
@@ -135,19 +130,16 @@
 
   /**
    * Parse strings like "60x12" or "77,1"
-   * @param {string} string
-   * @returns {[number, number]}
    */
-  const parseTuple = (string) => {
+  const parseTuple = (string: string): [number, number] => {
     const [a, b] = ("" + string).split(/[ ,x]/);
     return [+a || 0, +b || 0];
   };
 
   /**
-   * @param {string} size eg. "48x84"
-   * @returns {string}
+   * @param size eg. "48x84"
    */
-  const formatUnreliableSize = (size) =>
+  const formatUnreliableSize = (size: string): string =>
     Scratch.translate(
       {
         default: "{size} (unreliable)",
@@ -196,7 +188,7 @@
     "nwse-resize",
   ];
 
-  class MouseCursor {
+  class MouseCursor implements Scratch.Extension {
     constructor() {
       Scratch.vm.runtime.on("RUNTIME_DISPOSED", () => {
         this.setCur({
@@ -560,7 +552,7 @@
       };
     }
 
-    setCur(args) {
+    setCur(args: { cur: unknown }) {
       const newCursor = Scratch.Cast.toString(args.cur);
       // Prevent setting cursor to "url(...), default" from causing fetch.
       if (ALL_ALLOWED_CURSORS.includes(newCursor)) {
@@ -571,10 +563,13 @@
       }
     }
 
-    setCursorImage(args, util) {
-      const [maxWidth, maxHeight] = parseTuple(args.size).map((i) =>
-        Math.max(0, i)
-      );
+    setCursorImage(
+      args: { position: unknown; size: unknown },
+      util: VM.BlockUtility
+    ) {
+      const [maxWidth, maxHeight] = parseTuple(
+        Scratch.Cast.toString(args.size)
+      ).map((i) => Math.max(0, i));
 
       const currentCostume =
         util.target.getCostumes()[util.target.currentCostume];
@@ -589,9 +584,9 @@
       }
 
       if (encodedCostume) {
-        const [percentX, percentY] = parseTuple(args.position).map(
-          (i) => Math.max(0, Math.min(100, i)) / 100
-        );
+        const [percentX, percentY] = parseTuple(
+          Scratch.Cast.toString(args.position)
+        ).map((i) => Math.max(0, Math.min(100, i)) / 100);
         const x = percentX * encodedCostume.width;
         const y = percentY * encodedCostume.height;
 
diff --git a/extensions/gamepad.js b/extensions/gamepad.ts
similarity index 87%
rename from extensions/gamepad.js
rename to extensions/gamepad.ts
index e8f06ae2cd..c2b4806cb0 100644
--- a/extensions/gamepad.js
+++ b/extensions/gamepad.ts
@@ -7,7 +7,7 @@
 // Some parts of this scripts are based on or designed to be compatible-ish with:
 // https://arpruss.github.io/gamepad.js (MIT Licensed)
 
-(function (Scratch) {
+(function (Scratch: typeof globalThis.Scratch) {
   "use strict";
 
   // For joysticks
@@ -17,20 +17,18 @@
   // For triggers. Drift isn't so big of an issue with these.
   const BUTTON_DEADZONE = 0.05;
 
-  /**
-   * @typedef InternalGamepadState
-   * @property {string} id
-   * @property {Gamepad} realGamepad
-   * @property {number} timestamp
-   * @property {number[]} axisDirections
-   * @property {number[]} axisMagnitudes
-   * @property {number[]} axisValues
-   * @property {number[]} buttonValues
-   * @property {boolean[]} buttonPressed
-   */
+  interface InternalGamepadState {
+    id: string;
+    realGamepad: Gamepad;
+    timestamp: number;
+    axisDirections: number[];
+    axisMagnitudes: number[];
+    axisValues: number[];
+    buttonValues: number[];
+    buttonPressed: boolean[];
+  }
 
-  /** @type {Array<InternalGamepadState|null>} */
-  let gamepadState = [];
+  let gamepadState: Array<InternalGamepadState | null> = [];
 
   const updateState = () => {
     // In Firefox, the objects returned by getGamepads() change in the background, but in Chrome
@@ -46,8 +44,7 @@
         return null;
       }
 
-      /** @type {InternalGamepadState} */
-      const result = {
+      const result: InternalGamepadState = {
         id: gamepad.id,
         realGamepad: gamepad,
         timestamp: gamepad.timestamp,
@@ -104,12 +101,11 @@
   });
 
   /**
-   * @param {unknown} index 1-indexed index or 'any'
-   * @returns {InternalGamepadState[]}
+   * @param index 1-indexed index or 'any'
    */
-  const getGamepads = (index) => {
+  const getGamepads = (index: unknown): InternalGamepadState[] => {
     if (index === "any") {
-      return gamepadState.filter((i) => i);
+      return gamepadState.filter((i): i is InternalGamepadState => !!i);
     }
     const gamepad = gamepadState[Scratch.Cast.toNumber(index) - 1];
     if (gamepad) {
@@ -119,11 +115,13 @@
   };
 
   /**
-   * @param {InternalGamepadState} gamepad
-   * @param {unknown} buttonIndex 1-indexed index or 'any'
-   * @returns {boolean} false if button does not exist
+   * @param buttonIndex 1-indexed index or 'any'
+   * @returns false if button does not exist
    */
-  const isButtonPressed = (gamepad, buttonIndex) => {
+  const isButtonPressed = (
+    gamepad: InternalGamepadState,
+    buttonIndex: unknown
+  ): boolean => {
     if (buttonIndex === "any") {
       return gamepad.buttonPressed.some((i) => i);
     }
@@ -131,47 +129,49 @@
   };
 
   /**
-   * @param {InternalGamepadState} gamepad
-   * @param {unknown} buttonIndex 1-indexed index
-   * @returns {number} 0 if button does not exist
+   * @param buttonIndex 1-indexed index
+   * @returns 0 if button does not exist
    */
-  const getButtonValue = (gamepad, buttonIndex) => {
+  const getButtonValue = (
+    gamepad: InternalGamepadState,
+    buttonIndex: unknown
+  ): number => {
     const value = gamepad.buttonValues[Scratch.Cast.toNumber(buttonIndex) - 1];
     return value || 0;
   };
 
   /**
-   * @param {InternalGamepadState} gamepad
-   * @param {unknown} axisIndex 1-indexed index
-   * @returns {number} 0 if axis does not exist
+   * @param axisIndex 1-indexed index
+   * @returns 0 if axis does not exist
    */
-  const getAxisValue = (gamepad, axisIndex) => {
+  const getAxisValue = (
+    gamepad: InternalGamepadState,
+    axisIndex: unknown
+  ): number => {
     const axisValue = gamepad.axisValues[Scratch.Cast.toNumber(axisIndex) - 1];
     return axisValue || 0;
   };
 
-  /**
-   * @param {InternalGamepadState} gamepad
-   * @param {unknown} startIndex
-   */
-  const getAxisPairMagnitude = (gamepad, startIndex) => {
+  const getAxisPairMagnitude = (
+    gamepad: InternalGamepadState,
+    startIndex: unknown
+  ) => {
     const magnitude =
       gamepad.axisMagnitudes[Scratch.Cast.toNumber(startIndex) - 1];
     return magnitude || 0;
   };
 
-  /**
-   * @param {InternalGamepadState} gamepad
-   * @param {unknown} startIndex
-   */
-  const getAxisPairDirection = (gamepad, startIndex) => {
+  const getAxisPairDirection = (
+    gamepad: InternalGamepadState,
+    startIndex: unknown
+  ) => {
     const direction =
       gamepad.axisDirections[Scratch.Cast.toNumber(startIndex) - 1];
     return direction || 0;
   };
 
-  class GamepadExtension {
-    getInfo() {
+  class GamepadExtension implements Scratch.Extension {
+    getInfo(): Scratch.Info {
       return {
         id: "Gamepad",
         name: Scratch.translate("Gamepad"),
@@ -513,11 +513,11 @@
       };
     }
 
-    gamepadConnected({ pad }) {
+    gamepadConnected({ pad }: { pad: unknown }) {
       return getGamepads(pad).length > 0;
     }
 
-    buttonDown({ b, i }) {
+    buttonDown({ b, i }: { b: unknown; i: unknown }) {
       for (const gamepad of getGamepads(i)) {
         if (isButtonPressed(gamepad, b)) {
           return true;
@@ -526,7 +526,7 @@
       return false;
     }
 
-    buttonValue({ b, i }) {
+    buttonValue({ b, i }: { b: unknown; i: unknown }) {
       let greatestButton = 0;
       for (const gamepad of getGamepads(i)) {
         const value = getButtonValue(gamepad, b);
@@ -537,7 +537,7 @@
       return greatestButton;
     }
 
-    axisValue({ b, i }) {
+    axisValue({ b, i }: { b: unknown; i: unknown }) {
       let greatestAxis = 0;
       for (const gamepad of getGamepads(i)) {
         const axis = getAxisValue(gamepad, b);
@@ -548,7 +548,7 @@
       return greatestAxis;
     }
 
-    axisDirection({ axis, pad }) {
+    axisDirection({ axis, pad }: { axis: unknown; pad: unknown }) {
       let greatestMagnitude = 0;
       // by default sprites have direction 90 degrees, so that's a reasonable default
       let direction = 90;
@@ -571,11 +571,14 @@
       return direction;
     }
 
-    axisMagnitude({ axis, pad }) {
+    axisMagnitude({ axis, pad }: { axis: unknown; pad: unknown }) {
       let greatestMagnitude = 0;
       for (const gamepad of getGamepads(pad)) {
         const horizontalAxis = getAxisValue(gamepad, axis);
-        const verticalAxis = getAxisValue(gamepad, +axis + 1);
+        const verticalAxis = getAxisValue(
+          gamepad,
+          Scratch.Cast.toNumber(axis) + 1
+        );
         const magnitude = Math.sqrt(horizontalAxis ** 2 + verticalAxis ** 2);
         if (magnitude > greatestMagnitude) {
           greatestMagnitude = magnitude;
@@ -584,23 +587,21 @@
       return greatestMagnitude;
     }
 
-    rumble({ s, w, t, i }) {
+    rumble({ s, w, t, i }: { s: unknown; w: unknown; t: unknown; i: unknown }) {
       const gamepads = getGamepads(i);
       for (const { realGamepad } of gamepads) {
-        // @ts-ignore
         if (realGamepad.vibrationActuator) {
-          // @ts-ignore
           realGamepad.vibrationActuator.playEffect("dual-rumble", {
             startDelay: 0,
-            duration: t * 1000,
-            weakMagnitude: w,
-            strongMagnitude: s,
+            duration: Scratch.Cast.toNumber(t) * 1000,
+            weakMagnitude: Scratch.Cast.toNumber(w),
+            strongMagnitude: Scratch.Cast.toNumber(s),
           });
         }
       }
     }
 
-    setAxisDeadzone({ DEADZONE }) {
+    setAxisDeadzone({ DEADZONE }: { DEADZONE: unknown }) {
       axisDeadzone = Scratch.Cast.toNumber(DEADZONE);
       updateState();
     }
diff --git a/extensions/iframe.js b/extensions/iframe.ts
similarity index 91%
rename from extensions/iframe.js
rename to extensions/iframe.ts
index 3d4d281d45..a1db92ae09 100644
--- a/extensions/iframe.js
+++ b/extensions/iframe.ts
@@ -5,12 +5,11 @@
 // Context: "iframe" is an HTML element that lets websites embed other websites.
 // License: MIT AND MPL-2.0
 
-(function (Scratch) {
+(function (Scratch: typeof globalThis.Scratch) {
   "use strict";
 
-  /** @type {HTMLIFrameElement|null} */
-  let iframe = null;
-  let overlay = null;
+  let iframe: HTMLIFrameElement | null = null;
+  let overlay: RenderWebGL.Overlay | null = null;
 
   const featurePolicy = {
     accelerometer: "'none'",
@@ -54,10 +53,8 @@
   let height = -1; // negative means default
   let interactive = true;
   let resizeBehavior = "scale";
-  /** @type {string|number|boolean} */
-  let latestMessage = "";
-  /** @type {string|number|boolean} */
-  let latestParentMessage = "";
+  let latestMessage: string | number | boolean = "";
+  let latestParentMessage: string | number | boolean = "";
 
   const updateFrameAttributes = () => {
     if (!iframe) {
@@ -97,7 +94,7 @@
   const getOverlayMode = () =>
     resizeBehavior === "scale" ? "scale-centered" : "manual";
 
-  const createFrame = (src) => {
+  const createFrame = (src: string) => {
     iframe = document.createElement("iframe");
     iframe.style.width = "100%";
     iframe.style.height = "100%";
@@ -126,18 +123,14 @@
     }
   };
 
-  /** @param {unknown} data */
-  const normalizeMessage = (data) =>
+  const normalizeMessage = (data: unknown): string | number | boolean =>
     typeof data === "string" ||
     typeof data === "number" ||
     typeof data === "boolean"
       ? data
       : JSON.stringify(data);
 
-  /**
-   * @returns {Window|null}
-   */
-  const getParentWindow = () => {
+  const getParentWindow = (): Window | null => {
     // if no parent, window.parent is us. which is not useful
     if (window.parent !== window) {
       return window.parent;
@@ -162,8 +155,8 @@
 
   Scratch.vm.runtime.on("RUNTIME_DISPOSED", closeFrame);
 
-  class IframeExtension {
-    getInfo() {
+  class IframeExtension implements Scratch.Extension {
+    getInfo(): Scratch.Info {
       return {
         name: Scratch.translate("Iframe"),
         id: "iframe",
@@ -370,14 +363,15 @@
       };
     }
 
-    async display({ URL }) {
+    async display({ URL }: { URL: unknown }) {
       closeFrame();
-      if (await Scratch.canEmbed(URL)) {
-        createFrame(Scratch.Cast.toString(URL));
+      const url = Scratch.Cast.toString(URL);
+      if (await Scratch.canEmbed(url)) {
+        createFrame(url);
       }
     }
 
-    async displayHTML({ HTML }) {
+    async displayHTML({ HTML }: { HTML: unknown }) {
       closeFrame();
       const url = `data:text/html;,${encodeURIComponent(
         Scratch.Cast.toString(HTML)
@@ -403,7 +397,7 @@
       closeFrame();
     }
 
-    get({ MENU }) {
+    get({ MENU }: { MENU: unknown }) {
       MENU = Scratch.Cast.toString(MENU);
       if (MENU === "url") {
         if (iframe) return iframe.getAttribute("src");
@@ -427,32 +421,32 @@
       }
     }
 
-    setX({ X }) {
+    setX({ X }: { X: unknown }) {
       x = Scratch.Cast.toNumber(X);
       updateFrameAttributes();
     }
 
-    setY({ Y }) {
+    setY({ Y }: { Y: unknown }) {
       y = Scratch.Cast.toNumber(Y);
       updateFrameAttributes();
     }
 
-    setWidth({ WIDTH }) {
+    setWidth({ WIDTH }: { WIDTH: unknown }) {
       width = Scratch.Cast.toNumber(WIDTH);
       updateFrameAttributes();
     }
 
-    setHeight({ HEIGHT }) {
+    setHeight({ HEIGHT }: { HEIGHT: unknown }) {
       height = Scratch.Cast.toNumber(HEIGHT);
       updateFrameAttributes();
     }
 
-    setInteractive({ INTERACTIVE }) {
+    setInteractive({ INTERACTIVE }: { INTERACTIVE: unknown }) {
       interactive = Scratch.Cast.toBoolean(INTERACTIVE);
       updateFrameAttributes();
     }
 
-    setResize({ RESIZE }) {
+    setResize({ RESIZE }: { RESIZE: unknown }) {
       if (RESIZE === "scale" || RESIZE === "viewport") {
         resizeBehavior = RESIZE;
         if (overlay) {
@@ -463,13 +457,13 @@
       }
     }
 
-    sendMessage({ MESSAGE }) {
+    sendMessage({ MESSAGE }: { MESSAGE: unknown }) {
       if (iframe && iframe.contentWindow) {
         iframe.contentWindow.postMessage(MESSAGE, "*");
       }
     }
 
-    sendMessageParent({ MESSAGE }) {
+    sendMessageParent({ MESSAGE }: { MESSAGE: unknown }) {
       const parentWindow = getParentWindow();
       if (parentWindow) {
         parentWindow.postMessage(MESSAGE, "*");
diff --git a/extensions/runtime-options.js b/extensions/runtime-options.ts
similarity index 90%
rename from extensions/runtime-options.js
rename to extensions/runtime-options.ts
index 3ce5a68c6a..5f079f7d1f 100644
--- a/extensions/runtime-options.js
+++ b/extensions/runtime-options.ts
@@ -4,7 +4,7 @@
 // By: GarboMuffin
 // License: MIT AND MPL-2.0
 
-(function (Scratch) {
+(function (Scratch: typeof globalThis.Scratch) {
   "use strict";
 
   if (!Scratch.extensions.unsandboxed) {
@@ -23,18 +23,12 @@
   const STAGE_SIZE = "stage size";
   const USERNAME = "username";
 
-  /** @param {string} what */
-  const emitChanged = (what) =>
+  const emitChanged = (what: string) =>
     Scratch.vm.runtime.startHats("runtimeoptions_whenChange", {
       WHAT: what,
     });
 
-  /**
-   * @template T
-   * @param {T} obj
-   * @returns {T}
-   */
-  const shallowCopy = (obj) => Object.assign({}, obj);
+  const shallowCopy = <T>(obj: T): T => Object.assign({}, obj);
 
   let previousRuntimeOptions = shallowCopy(Scratch.vm.runtime.runtimeOptions);
 
@@ -60,7 +54,10 @@
   Scratch.vm.on("STAGE_SIZE_CHANGED", () => emitChanged(STAGE_SIZE));
 
   const originalPostData = Scratch.vm.runtime.ioDevices.userData.postData;
-  Scratch.vm.runtime.ioDevices.userData.postData = function (data) {
+  Scratch.vm.runtime.ioDevices.userData.postData = function (
+    this: VM.UserData,
+    data: VM.UserDataData
+  ) {
     const newUsername = data.username !== this._username;
     originalPostData.call(this, data);
     if (newUsername) {
@@ -68,8 +65,8 @@
     }
   };
 
-  class RuntimeOptions {
-    getInfo() {
+  class RuntimeOptions implements Scratch.Extension {
+    getInfo(): Scratch.Info {
       return {
         id: "runtimeoptions",
         name: Scratch.translate("Runtime Options"),
@@ -334,7 +331,7 @@
       };
     }
 
-    getEnabled({ thing }) {
+    getEnabled({ thing }: { thing: unknown }) {
       if (thing === TURBO_MODE) {
         return Scratch.vm.runtime.turboMode;
       } else if (thing === INTERPOLATION) {
@@ -349,23 +346,23 @@
       return false;
     }
 
-    setEnabled({ thing, enabled }) {
-      enabled = Scratch.Cast.toBoolean(enabled);
+    setEnabled({ thing, enabled }: { thing: unknown; enabled: unknown }) {
+      const isEnabled = Scratch.Cast.toBoolean(enabled);
 
       if (thing === TURBO_MODE) {
-        Scratch.vm.setTurboMode(enabled);
+        Scratch.vm.setTurboMode(isEnabled);
       } else if (thing === INTERPOLATION) {
-        Scratch.vm.setInterpolation(enabled);
+        Scratch.vm.setInterpolation(isEnabled);
       } else if (thing === REMOVE_FENCING) {
         Scratch.vm.setRuntimeOptions({
-          fencing: !enabled,
+          fencing: !isEnabled,
         });
       } else if (thing === REMOVE_MISC_LIMITS) {
         Scratch.vm.setRuntimeOptions({
-          miscLimits: !enabled,
+          miscLimits: !isEnabled,
         });
       } else if (thing === HIGH_QUALITY_PEN) {
-        Scratch.renderer.setUseHighQualityRender(enabled);
+        Scratch.renderer.setUseHighQualityRender(isEnabled);
       }
     }
 
@@ -373,22 +370,20 @@
       return Scratch.vm.runtime.frameLoop.framerate;
     }
 
-    setFramerate({ fps }) {
-      fps = Scratch.Cast.toNumber(fps);
-      Scratch.vm.setFramerate(fps);
+    setFramerate({ fps }: { fps: unknown }) {
+      Scratch.vm.setFramerate(Scratch.Cast.toNumber(fps));
     }
 
     getCloneLimit() {
       return Scratch.vm.runtime.runtimeOptions.maxClones;
     }
-    setCloneLimit({ limit }) {
-      limit = Scratch.Cast.toNumber(limit);
+    setCloneLimit({ limit }: { limit: unknown }) {
       Scratch.vm.setRuntimeOptions({
-        maxClones: limit,
+        maxClones: Scratch.Cast.toNumber(limit),
       });
     }
 
-    getDimension({ dimension }) {
+    getDimension({ dimension }: { dimension: unknown }) {
       if (dimension === "width") {
         return Scratch.vm.runtime.stageWidth;
       } else if (dimension === "height") {
@@ -397,13 +392,14 @@
       return 0;
     }
 
-    setDimensions({ width, height }) {
-      width = Scratch.Cast.toNumber(width);
-      height = Scratch.Cast.toNumber(height);
-      Scratch.vm.setStageSize(width, height);
+    setDimensions({ width, height }: { width: unknown; height: unknown }) {
+      Scratch.vm.setStageSize(
+        Scratch.Cast.toNumber(width),
+        Scratch.Cast.toNumber(height)
+      );
     }
 
-    setUsername({ username }) {
+    setUsername({ username }: { username: unknown }) {
       Scratch.vm.postIOData("userData", {
         username: Scratch.Cast.toString(username),
       });
diff --git a/package-lock.json b/package-lock.json
index 88c4cb657f..e9126aaac4 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -255,8 +255,7 @@
     },
     "node_modules/@turbowarp/types": {
       "version": "0.0.14",
-      "resolved": "git+ssh://git@github.com/TurboWarp/types-tw.git#938c3ab84e4dd859f6f0646cdf7ed5dd1a94f622",
-      "integrity": "sha512-oDu7BzT6YriGTaX7k8UmPuTiC/Cr2dmhwoWRRCMvzaOSDNZ4E45DM5B9uO477eQFcSSiUzW+ELhphD4+rNFKFQ==",
+      "resolved": "git+ssh://git@github.com/TurboWarp/types-tw.git#783efabc66fc60e637d800658c1e54035dc83044",
       "license": "Apache-2.0"
     },
     "node_modules/@types/estree": {
diff --git a/tsconfig.json b/tsconfig.json
index 07ec64f2ec..47540a528b 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -4,6 +4,9 @@
     "noEmit": true,
     "allowJs": true,
     "checkJs": true,
+    "removeComments": false,
+    "sourceMap": true,
+    "inlineSources": true,
     "paths": {
       // See https://github.com/turboWarp/types#using-from-npm
       "scratch-vm": ["./node_modules/@turbowarp/types/index.d.ts"],