diff --git a/cli/js/body.ts b/cli/js/body.ts
index fc12efa8af..ed21fa0ecf 100644
--- a/cli/js/body.ts
+++ b/cli/js/body.ts
@@ -143,9 +143,10 @@ export class Body implements domTypes.Body {
       this._stream = this._bodySource;
     }
     if (typeof this._bodySource === "string") {
+      const bodySource = this._bodySource;
       this._stream = new ReadableStream({
         start(controller: ReadableStreamController): void {
-          controller.enqueue(this._bodySource);
+          controller.enqueue(bodySource);
           controller.close();
         }
       });
diff --git a/cli/js/compiler.ts b/cli/js/compiler.ts
index 775277cdd4..1779f76c36 100644
--- a/cli/js/compiler.ts
+++ b/cli/js/compiler.ts
@@ -285,7 +285,7 @@ async function processImports(
   referrer = ""
 ): Promise<SourceFileJson[]> {
   if (!specifiers.length) {
-    return;
+    return [];
   }
   const sources = specifiers.map(([, moduleSpecifier]) => moduleSpecifier);
   const sourceFiles = await fetchSourceFiles(sources, referrer);
@@ -385,6 +385,8 @@ class Host implements ts.CompilerHost {
   private readonly _options: ts.CompilerOptions = {
     allowJs: true,
     allowNonTsExtensions: true,
+    // TODO(#3324) Enable strict mode for user code.
+    // strict: true,
     checkJs: false,
     esModuleInterop: true,
     module: ts.ModuleKind.ESNext,
diff --git a/cli/js/lib.deno_runtime.d.ts b/cli/js/lib.deno_runtime.d.ts
index 87be1c3a09..c7c4039879 100644
--- a/cli/js/lib.deno_runtime.d.ts
+++ b/cli/js/lib.deno_runtime.d.ts
@@ -2858,4 +2858,10 @@ declare namespace WebAssembly {
   }
 }
 
-/* eslint-enable @typescript-eslint/no-unused-vars, @typescript-eslint/no-explicit-any */
+// Catch-all for JSX elements.
+// See https://www.typescriptlang.org/docs/handbook/jsx.html#intrinsic-elements
+declare namespace JSX {
+  interface IntrinsicElements {
+    [elemName: string]: any;
+  }
+}
diff --git a/cli/js/os.ts b/cli/js/os.ts
index 89e5652cb5..4e17e2030b 100644
--- a/cli/js/os.ts
+++ b/cli/js/os.ts
@@ -95,7 +95,8 @@ export function start(preserveDenoNamespace = true, source?: string): Start {
   for (const [name, opId] of Object.entries(ops)) {
     const opName = `OP_${name.toUpperCase()}`;
     // Assign op ids to actual variables
-    dispatch[opName] = opId;
+    // TODO(ry) This type casting is gross and should be fixed.
+    ((dispatch as unknown) as { [key: string]: number })[opName] = opId;
   }
   // First we send an empty `Start` message to let the privileged side know we
   // are ready. The response should be a `StartRes` message containing the CLI
diff --git a/cli/js/streams/readable-internals.ts b/cli/js/streams/readable-internals.ts
index 36f4223d79..67f5a69b1d 100644
--- a/cli/js/streams/readable-internals.ts
+++ b/cli/js/streams/readable-internals.ts
@@ -259,7 +259,7 @@ export function isReadableStreamLocked<OutputType>(
 
 export function readableStreamGetNumReadIntoRequests<OutputType>(
   stream: SDReadableStream<OutputType>
-): number | undefined {
+): number {
   // TODO remove the "as unknown" cast
   // This is in to workaround a compiler error
   // error TS2352: Conversion of type 'SDReadableStreamReader<OutputType>' to type 'SDReadableStreamBYOBReader' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
diff --git a/cli/js/url.ts b/cli/js/url.ts
index 2017bb40b7..489d8d0454 100644
--- a/cli/js/url.ts
+++ b/cli/js/url.ts
@@ -156,7 +156,7 @@ export class URL {
       "search"
     ];
     const objectString = keys
-      .map((key: string) => `${key}: "${this[key] || ""}"`)
+      .map((key: string) => `${key}: "${this[key as keyof this] || ""}"`)
       .join(", ");
     return `URL { ${objectString} }`;
   }
diff --git a/deno_typescript/lib.deno_core.d.ts b/deno_typescript/lib.deno_core.d.ts
index c3a20eb08b..5496655431 100644
--- a/deno_typescript/lib.deno_core.d.ts
+++ b/deno_typescript/lib.deno_core.d.ts
@@ -21,7 +21,7 @@ interface EvalErrorInfo {
 }
 
 declare interface DenoCore {
-  print(s: string, isErr?: boolean);
+  print(s: string, isErr?: boolean): void;
   dispatch(
     opId: number,
     control: Uint8Array,
diff --git a/deno_typescript/lib.rs b/deno_typescript/lib.rs
index 56072ebdc4..e53a4243d8 100644
--- a/deno_typescript/lib.rs
+++ b/deno_typescript/lib.rs
@@ -127,6 +127,7 @@ pub fn compile_bundle(
 
   let config_json = serde_json::json!({
     "compilerOptions": {
+      "strict": true,
       "declaration": true,
       "lib": ["esnext"],
       "module": "amd",
diff --git a/std/node/fs.ts b/std/node/fs.ts
index 01b9e23989..539916c9ee 100644
--- a/std/node/fs.ts
+++ b/std/node/fs.ts
@@ -1,7 +1,14 @@
-import { notImplemented, intoCallbackAPIWithIntercept } from "./_utils.ts";
+import {
+  notImplemented,
+  intoCallbackAPIWithIntercept,
+  MaybeEmpty
+} from "./_utils.ts";
 const { readFile: denoReadFile, readFileSync: denoReadFileSync } = Deno;
 
-type ReadFileCallback = (err: Error | null, data: string | Uint8Array) => void;
+type ReadFileCallback = (
+  err: MaybeEmpty<Error>,
+  data: MaybeEmpty<string | Uint8Array>
+) => void;
 
 interface ReadFileOptions {
   encoding?: string | null;