Enable TS strict mode by default (#3899)

Fixes #3324 

Co-authored-by: Kitson Kelly <me@kitsonkelly.com>

27 files changed, 162 insertions, 84 deletions

diff --git a/cli/js/body_test.ts b/cli/js/body_test.ts
index 2ac4d1314..d7f5c6d61 100644
--- a/cli/js/body_test.ts
+++ b/cli/js/body_test.ts
@@ -45,7 +45,7 @@ testPerm({ net: true }, async function bodyMultipartFormData(): Promise<void> {
 
   const formData = await body.formData();
   assert(formData.has("field_1"));
-  assertEquals(formData.get("field_1").toString(), "value_1 \r\n");
+  assertEquals(formData.get("field_1")!.toString(), "value_1 \r\n");
   assert(formData.has("field_2"));
 });
 
@@ -62,7 +62,7 @@ testPerm({ net: true }, async function bodyURLEncodedFormData(): Promise<void> {
 
   const formData = await body.formData();
   assert(formData.has("field_1"));
-  assertEquals(formData.get("field_1").toString(), "Hi");
+  assertEquals(formData.get("field_1")!.toString(), "Hi");
   assert(formData.has("field_2"));
-  assertEquals(formData.get("field_2").toString(), "<Deno>");
+  assertEquals(formData.get("field_2")!.toString(), "<Deno>");
 });
diff --git a/cli/js/buffer_test.ts b/cli/js/buffer_test.ts
index 4b5701e54..72c0cab4b 100644
--- a/cli/js/buffer_test.ts
+++ b/cli/js/buffer_test.ts
@@ -3,7 +3,7 @@
 // This code has been ported almost directly from Go's src/bytes/buffer_test.go
 // Copyright 2009 The Go Authors. All rights reserved. BSD license.
 // https://github.com/golang/go/blob/master/LICENSE
-import { assertEquals, test } from "./test_util.ts";
+import { assert, assertEquals, test } from "./test_util.ts";
 
 const { Buffer, readAll, readAllSync, writeAll, writeAllSync } = Deno;
 type Buffer = Deno.Buffer;
@@ -78,12 +78,16 @@ function repeat(c: string, bytes: number): Uint8Array {
 
 test(function bufferNewBuffer(): void {
   init();
+  assert(testBytes);
+  assert(testString);
   const buf = new Buffer(testBytes.buffer as ArrayBuffer);
   check(buf, testString);
 });
 
 test(async function bufferBasicOperations(): Promise<void> {
   init();
+  assert(testBytes);
+  assert(testString);
   const buf = new Buffer();
   for (let i = 0; i < 5; i++) {
     check(buf, "");
@@ -134,8 +138,8 @@ test(async function bufferLargeByteWrites(): Promise<void> {
   const buf = new Buffer();
   const limit = 9;
   for (let i = 3; i < limit; i += 3) {
-    const s = await fillBytes(buf, "", 5, testBytes);
-    await empty(buf, s, new Uint8Array(Math.floor(testString.length / i)));
+    const s = await fillBytes(buf, "", 5, testBytes!);
+    await empty(buf, s, new Uint8Array(Math.floor(testString!.length / i)));
   }
   check(buf, "");
 });
@@ -161,6 +165,8 @@ test(async function bufferTooLargeByteWrites(): Promise<void> {
 
 test(async function bufferLargeByteReads(): Promise<void> {
   init();
+  assert(testBytes);
+  assert(testString);
   const buf = new Buffer();
   for (let i = 3; i < 30; i += 3) {
     const n = Math.floor(testBytes.byteLength / i);
@@ -177,6 +183,8 @@ test(function bufferCapWithPreallocatedSlice(): void {
 
 test(async function bufferReadFrom(): Promise<void> {
   init();
+  assert(testBytes);
+  assert(testString);
   const buf = new Buffer();
   for (let i = 3; i < 30; i += 3) {
     const s = await fillBytes(
@@ -194,6 +202,8 @@ test(async function bufferReadFrom(): Promise<void> {
 
 test(async function bufferReadFromSync(): Promise<void> {
   init();
+  assert(testBytes);
+  assert(testString);
   const buf = new Buffer();
   for (let i = 3; i < 30; i += 3) {
     const s = await fillBytes(
@@ -236,6 +246,7 @@ test(async function bufferTestGrow(): Promise<void> {
 
 test(async function testReadAll(): Promise<void> {
   init();
+  assert(testBytes);
   const reader = new Buffer(testBytes.buffer as ArrayBuffer);
   const actualBytes = await readAll(reader);
   assertEquals(testBytes.byteLength, actualBytes.byteLength);
@@ -246,6 +257,7 @@ test(async function testReadAll(): Promise<void> {
 
 test(function testReadAllSync(): void {
   init();
+  assert(testBytes);
   const reader = new Buffer(testBytes.buffer as ArrayBuffer);
   const actualBytes = readAllSync(reader);
   assertEquals(testBytes.byteLength, actualBytes.byteLength);
@@ -256,6 +268,7 @@ test(function testReadAllSync(): void {
 
 test(async function testWriteAll(): Promise<void> {
   init();
+  assert(testBytes);
   const writer = new Buffer();
   await writeAll(writer, testBytes);
   const actualBytes = writer.bytes();
@@ -267,6 +280,7 @@ test(async function testWriteAll(): Promise<void> {
 
 test(function testWriteAllSync(): void {
   init();
+  assert(testBytes);
   const writer = new Buffer();
   writeAllSync(writer, testBytes);
   const actualBytes = writer.bytes();
diff --git a/cli/js/chmod_test.ts b/cli/js/chmod_test.ts
index 3ecb4256a..b3b0a2ae2 100644
--- a/cli/js/chmod_test.ts
+++ b/cli/js/chmod_test.ts
@@ -1,5 +1,5 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import { testPerm, assertEquals } from "./test_util.ts";
+import { testPerm, assert, assertEquals } from "./test_util.ts";
 
 const isNotWindows = Deno.build.os !== "win";
 
@@ -16,6 +16,7 @@ testPerm({ read: true, write: true }, function chmodSyncSuccess(): void {
   // Check success when not on windows
   if (isNotWindows) {
     const fileInfo = Deno.statSync(filename);
+    assert(fileInfo.mode);
     assertEquals(fileInfo.mode & 0o777, 0o777);
   }
 });
@@ -35,14 +36,17 @@ if (isNotWindows) {
       Deno.symlinkSync(filename, symlinkName);
 
       let symlinkInfo = Deno.lstatSync(symlinkName);
+      assert(symlinkInfo.mode);
       const symlinkMode = symlinkInfo.mode & 0o777; // platform dependent
 
       Deno.chmodSync(symlinkName, 0o777);
 
       // Change actual file mode, not symlink
       const fileInfo = Deno.statSync(filename);
+      assert(fileInfo.mode);
       assertEquals(fileInfo.mode & 0o777, 0o777);
       symlinkInfo = Deno.lstatSync(symlinkName);
+      assert(symlinkInfo.mode);
       assertEquals(symlinkInfo.mode & 0o777, symlinkMode);
     }
   );
@@ -86,6 +90,7 @@ testPerm({ read: true, write: true }, async function chmodSuccess(): Promise<
   // Check success when not on windows
   if (isNotWindows) {
     const fileInfo = Deno.statSync(filename);
+    assert(fileInfo.mode);
     assertEquals(fileInfo.mode & 0o777, 0o777);
   }
 });
@@ -105,14 +110,17 @@ if (isNotWindows) {
       Deno.symlinkSync(filename, symlinkName);
 
       let symlinkInfo = Deno.lstatSync(symlinkName);
+      assert(symlinkInfo.mode);
       const symlinkMode = symlinkInfo.mode & 0o777; // platform dependent
 
       await Deno.chmod(symlinkName, 0o777);
 
       // Just change actual file mode, not symlink
       const fileInfo = Deno.statSync(filename);
+      assert(fileInfo.mode);
       assertEquals(fileInfo.mode & 0o777, 0o777);
       symlinkInfo = Deno.lstatSync(symlinkName);
+      assert(symlinkInfo.mode);
       assertEquals(symlinkInfo.mode & 0o777, symlinkMode);
     }
   );
diff --git a/cli/js/compiler_host.ts b/cli/js/compiler_host.ts
index 8f19eb326..d44bc7a03 100644
--- a/cli/js/compiler_host.ts
+++ b/cli/js/compiler_host.ts
@@ -49,8 +49,7 @@ export const defaultBundlerOptions: ts.CompilerOptions = {
 export const defaultCompileOptions: ts.CompilerOptions = {
   allowJs: true,
   allowNonTsExtensions: true,
-  // TODO(#3324) Enable strict mode for user code.
-  // strict: true,
+  strict: true,
   checkJs: false,
   esModuleInterop: true,
   module: ts.ModuleKind.ESNext,
diff --git a/cli/js/console_test.ts b/cli/js/console_test.ts
index b80dd8284..d9492cf2d 100644
--- a/cli/js/console_test.ts
+++ b/cli/js/console_test.ts
@@ -14,8 +14,8 @@ const customInspect = Deno.symbols.customInspect;
 const {
   Console,
   stringifyArgs
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
-} = Deno[Deno.symbols.internal] as any;
+  // @ts-ignore TypeScript (as of 3.7) does not support indexing namespaces by symbol
+} = Deno[Deno.symbols.internal];
 
 function stringify(...args: unknown[]): string {
   return stringifyArgs(args).replace(/\n$/, "");
@@ -306,6 +306,7 @@ test(function consoleTestCallToStringOnLabel(): void {
   for (const method of methods) {
     let hasCalled = false;
 
+    // @ts-ignore
     console[method]({
       toString(): void {
         hasCalled = true;
@@ -451,6 +452,7 @@ test(function consoleGroup(): void {
 // console.group with console.warn test
 test(function consoleGroupWarn(): void {
   mockConsole((console, _out, _err, both): void => {
+    assert(both);
     console.warn("1");
     console.group();
     console.warn("2");
@@ -694,6 +696,7 @@ test(function consoleDirXml(): void {
 test(function consoleTrace(): void {
   mockConsole((console, _out, err): void => {
     console.trace("%s", "custom message");
+    assert(err);
     assert(err.toString().includes("Trace: custom message"));
   });
 });
diff --git a/cli/js/error_stack_test.ts b/cli/js/error_stack_test.ts
index e4e44c77f..12755c166 100644
--- a/cli/js/error_stack_test.ts
+++ b/cli/js/error_stack_test.ts
@@ -1,8 +1,8 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { test, assert } from "./test_util.ts";
 
-// eslint-disable-next-line @typescript-eslint/no-explicit-any
-const { setPrepareStackTrace } = Deno[Deno.symbols.internal] as any;
+// @ts-ignore TypeScript (as of 3.7) does not support indexing namespaces by symbol
+const { setPrepareStackTrace } = Deno[Deno.symbols.internal];
 
 interface CallSite {
   getThis(): unknown;
diff --git a/cli/js/event_target_test.ts b/cli/js/event_target_test.ts
index aead97ed3..34d42d014 100644
--- a/cli/js/event_target_test.ts
+++ b/cli/js/event_target_test.ts
@@ -4,8 +4,11 @@ import { test, assertEquals } from "./test_util.ts";
 test(function addEventListenerTest(): void {
   const document = new EventTarget();
 
+  // @ts-ignore tests ignoring the type system for resilience
   assertEquals(document.addEventListener("x", null, false), undefined);
+  // @ts-ignore
   assertEquals(document.addEventListener("x", null, true), undefined);
+  // @ts-ignore
   assertEquals(document.addEventListener("x", null), undefined);
 });
 
@@ -14,7 +17,7 @@ test(function constructedEventTargetCanBeUsedAsExpected(): void {
   const event = new Event("foo", { bubbles: true, cancelable: false });
   let callCount = 0;
 
-  const listener = (e): void => {
+  const listener = (e: Event): void => {
     assertEquals(e, event);
     ++callCount;
   };
@@ -34,11 +37,19 @@ test(function constructedEventTargetCanBeUsedAsExpected(): void {
 
 test(function anEventTargetCanBeSubclassed(): void {
   class NicerEventTarget extends EventTarget {
-    on(type, callback?, options?): void {
+    on(
+      type: string,
+      callback: (e: Event) => void | null,
+      options?: __domTypes.AddEventListenerOptions
+    ): void {
       this.addEventListener(type, callback, options);
     }
 
-    off(type, callback?, options?): void {
+    off(
+      type: string,
+      callback: (e: Event) => void | null,
+      options?: __domTypes.EventListenerOptions
+    ): void {
       this.removeEventListener(type, callback, options);
     }
   }
@@ -60,8 +71,11 @@ test(function anEventTargetCanBeSubclassed(): void {
 
 test(function removingNullEventListenerShouldSucceed(): void {
   const document = new EventTarget();
+  // @ts-ignore
   assertEquals(document.removeEventListener("x", null, false), undefined);
+  // @ts-ignore
   assertEquals(document.removeEventListener("x", null, true), undefined);
+  // @ts-ignore
   assertEquals(document.removeEventListener("x", null), undefined);
 });
 
@@ -70,7 +84,7 @@ test(function constructedEventTargetUseObjectPrototype(): void {
   const event = new Event("toString", { bubbles: true, cancelable: false });
   let callCount = 0;
 
-  const listener = (e): void => {
+  const listener = (e: Event): void => {
     assertEquals(e, event);
     ++callCount;
   };
diff --git a/cli/js/event_test.ts b/cli/js/event_test.ts
index affd979ee..2ff23daf9 100644
--- a/cli/js/event_test.ts
+++ b/cli/js/event_test.ts
@@ -1,5 +1,5 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import { test, assertEquals, assertNotEquals } from "./test_util.ts";
+import { test, assertEquals, assert } from "./test_util.ts";
 
 test(function eventInitializedWithType(): void {
   const type = "click";
@@ -70,7 +70,8 @@ test(function eventPreventDefaultSuccess(): void {
 });
 
 test(function eventInitializedWithNonStringType(): void {
-  const type = undefined;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const type: any = undefined;
   const event = new Event(type);
 
   assertEquals(event.isTrusted, false);
@@ -84,12 +85,12 @@ test(function eventInitializedWithNonStringType(): void {
 // ref https://github.com/web-platform-tests/wpt/blob/master/dom/events/Event-isTrusted.any.js
 test(function eventIsTrusted(): void {
   const desc1 = Object.getOwnPropertyDescriptor(new Event("x"), "isTrusted");
-  assertNotEquals(desc1, undefined);
+  assert(desc1);
   assertEquals(typeof desc1.get, "function");
 
   const desc2 = Object.getOwnPropertyDescriptor(new Event("x"), "isTrusted");
-  assertNotEquals(desc2, undefined);
-  assertEquals(typeof desc2.get, "function");
+  assert(desc2);
+  assertEquals(typeof desc2!.get, "function");
 
-  assertEquals(desc1.get, desc2.get);
+  assertEquals(desc1!.get, desc2!.get);
 });
diff --git a/cli/js/fetch_test.ts b/cli/js/fetch_test.ts
index 2b3d03a38..c63d05501 100644
--- a/cli/js/fetch_test.ts
+++ b/cli/js/fetch_test.ts
@@ -54,7 +54,7 @@ testPerm({ net: true }, async function fetchHeaders(): Promise<void> {
   const response = await fetch("http://localhost:4545/cli/tests/fixture.json");
   const headers = response.headers;
   assertEquals(headers.get("Content-Type"), "application/json");
-  assert(headers.get("Server").startsWith("SimpleHTTP"));
+  assert(headers.get("Server")!.startsWith("SimpleHTTP"));
 });
 
 testPerm({ net: true }, async function fetchBlob(): Promise<void> {
@@ -93,10 +93,10 @@ testPerm({ net: true }, async function responseClone(): Promise<void> {
   assert(response !== response1);
   assertEquals(response.status, response1.status);
   assertEquals(response.statusText, response1.statusText);
-  const ab = await response.arrayBuffer();
-  const ab1 = await response1.arrayBuffer();
-  for (let i = 0; i < ab.byteLength; i++) {
-    assertEquals(ab[i], ab1[i]);
+  const u8a = new Uint8Array(await response.arrayBuffer());
+  const u8a1 = new Uint8Array(await response1.arrayBuffer());
+  for (let i = 0; i < u8a.byteLength; i++) {
+    assertEquals(u8a[i], u8a1[i]);
   }
 });
 
@@ -119,7 +119,7 @@ testPerm({ net: true }, async function fetchMultipartFormDataSuccess(): Promise<
   );
   const formData = await response.formData();
   assert(formData.has("field_1"));
-  assertEquals(formData.get("field_1").toString(), "value_1 \r\n");
+  assertEquals(formData.get("field_1")!.toString(), "value_1 \r\n");
   assert(formData.has("field_2"));
   /* TODO(ry) Re-enable this test once we bring back the global File type.
   const file = formData.get("field_2") as File;
@@ -136,9 +136,9 @@ testPerm(
     );
     const formData = await response.formData();
     assert(formData.has("field_1"));
-    assertEquals(formData.get("field_1").toString(), "Hi");
+    assertEquals(formData.get("field_1")!.toString(), "Hi");
     assert(formData.has("field_2"));
-    assertEquals(formData.get("field_2").toString(), "<Deno>");
+    assertEquals(formData.get("field_2")!.toString(), "<Deno>");
   }
 );
 
@@ -179,7 +179,7 @@ testPerm({ net: true }, async function fetchInitStringBody(): Promise<void> {
   });
   const text = await response.text();
   assertEquals(text, data);
-  assert(response.headers.get("content-type").startsWith("text/plain"));
+  assert(response.headers.get("content-type")!.startsWith("text/plain"));
 });
 
 testPerm({ net: true }, async function fetchRequestInitStringBody(): Promise<
@@ -220,7 +220,7 @@ testPerm({ net: true }, async function fetchInitURLSearchParamsBody(): Promise<
   assertEquals(text, data);
   assert(
     response.headers
-      .get("content-type")
+      .get("content-type")!
       .startsWith("application/x-www-form-urlencoded")
   );
 });
@@ -236,7 +236,7 @@ testPerm({ net: true }, async function fetchInitBlobBody(): Promise<void> {
   });
   const text = await response.text();
   assertEquals(text, data);
-  assert(response.headers.get("content-type").startsWith("text/javascript"));
+  assert(response.headers.get("content-type")!.startsWith("text/javascript"));
 });
 
 testPerm({ net: true }, async function fetchUserAgent(): Promise<void> {
diff --git a/cli/js/file_test.ts b/cli/js/file_test.ts
index 2d009c0c2..8fc37f701 100644
--- a/cli/js/file_test.ts
+++ b/cli/js/file_test.ts
@@ -1,7 +1,8 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 import { test, assert, assertEquals } from "./test_util.ts";
 
-function testFirstArgument(arg1, expectedSize): void {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function testFirstArgument(arg1: any[], expectedSize: number): void {
   const file = new File(arg1, "name");
   assert(file instanceof File);
   assertEquals(file.name, "name");
@@ -76,7 +77,8 @@ test(function fileObjectInFileBits(): void {
   testFirstArgument([{}], 15);
 });
 
-function testSecondArgument(arg2, expectedFileName): void {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function testSecondArgument(arg2: any, expectedFileName: string): void {
   const file = new File(["bits"], arg2);
   assert(file instanceof File);
   assertEquals(file.name, expectedFileName);
diff --git a/cli/js/files_test.ts b/cli/js/files_test.ts
index 8f4beb085..03e4d00e9 100644
--- a/cli/js/files_test.ts
+++ b/cli/js/files_test.ts
@@ -157,6 +157,7 @@ testPerm({ write: true }, async function writeNullBufferFailure(): Promise<
   // writing null should throw an error
   let err;
   try {
+    // @ts-ignore
     await file.write(null);
   } catch (e) {
     err = e;
@@ -182,6 +183,7 @@ testPerm(
     // reading file into null buffer should throw an error
     let err;
     try {
+      // @ts-ignore
       await file.read(null);
     } catch (e) {
       err = e;
diff --git a/cli/js/form_data_test.ts b/cli/js/form_data_test.ts
index a40326fba..10147d8b9 100644
--- a/cli/js/form_data_test.ts
+++ b/cli/js/form_data_test.ts
@@ -36,7 +36,9 @@ test(function formDataParamsGetSuccess(): void {
   formData.append("a", "true");
   formData.append("b", "false");
   formData.append("a", "null");
+  // @ts-ignore
   formData.append("d", undefined);
+  // @ts-ignore
   formData.append("e", null);
   assertEquals(formData.get("a"), "true");
   assertEquals(formData.get("b"), "false");
@@ -63,8 +65,10 @@ test(function formDataParamsSetSuccess(): void {
   assertEquals(formData.getAll("b"), ["false"]);
   formData.set("a", "false");
   assertEquals(formData.getAll("a"), ["false"]);
+  // @ts-ignore
   formData.set("d", undefined);
   assertEquals(formData.get("d"), "undefined");
+  // @ts-ignore
   formData.set("e", null);
   assertEquals(formData.get("e"), "null");
 });
@@ -101,15 +105,22 @@ test(function formDataParamsForEachSuccess(): void {
 });
 
 test(function formDataParamsArgumentsCheck(): void {
-  const methodRequireOneParam = ["delete", "getAll", "get", "has", "forEach"];
+  const methodRequireOneParam = [
+    "delete",
+    "getAll",
+    "get",
+    "has",
+    "forEach"
+  ] as const;
 
-  const methodRequireTwoParams = ["append", "set"];
+  const methodRequireTwoParams = ["append", "set"] as const;
 
   methodRequireOneParam.forEach((method): void => {
     const formData = new FormData();
     let hasThrown = 0;
     let errMsg = "";
     try {
+      // @ts-ignore
       formData[method]();
       hasThrown = 1;
     } catch (err) {
@@ -133,6 +144,7 @@ test(function formDataParamsArgumentsCheck(): void {
     let errMsg = "";
 
     try {
+      // @ts-ignore
       formData[method]();
       hasThrown = 1;
     } catch (err) {
@@ -152,6 +164,7 @@ test(function formDataParamsArgumentsCheck(): void {
     hasThrown = 0;
     errMsg = "";
     try {
+      // @ts-ignore
       formData[method]("foo");
       hasThrown = 1;
     } catch (err) {
diff --git a/cli/js/headers_test.ts b/cli/js/headers_test.ts
index 15e1191f5..65e04ada3 100644
--- a/cli/js/headers_test.ts
+++ b/cli/js/headers_test.ts
@@ -2,8 +2,8 @@
 import { test, assert, assertEquals } from "./test_util.ts";
 const {
   stringifyArgs
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
-} = Deno[Deno.symbols.internal] as any;
+  // @ts-ignore TypeScript (as of 3.7) does not support indexing namespaces by symbol
+} = Deno[Deno.symbols.internal];
 
 // Logic heavily copied from web-platform-tests, make
 // sure pass mostly header basic test
@@ -13,6 +13,7 @@ test(function newHeaderTest(): void {
   new Headers(undefined);
   new Headers({});
   try {
+    // @ts-ignore
     new Headers(null);
   } catch (e) {
     assertEquals(
@@ -22,14 +23,16 @@ test(function newHeaderTest(): void {
   }
 });
 
-const headerDict = {
+const headerDict: Record<string, string> = {
   name1: "value1",
   name2: "value2",
   name3: "value3",
+  // @ts-ignore
   name4: undefined,
   "Content-Type": "value4"
 };
-const headerSeq = [];
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const headerSeq: any[] = [];
 for (const name in headerDict) {
   headerSeq.push([name, headerDict[name]]);
 }
@@ -133,7 +136,7 @@ test(function headerValuesSuccess(): void {
   }
 });
 
-const headerEntriesDict = {
+const headerEntriesDict: Record<string, string> = {
   name1: "value1",
   Name2: "value2",
   name: "value3",
@@ -261,6 +264,7 @@ test(function headerParamsArgumentsCheck(): void {
     let hasThrown = 0;
     let errMsg = "";
     try {
+      // @ts-ignore
       headers[method]();
       hasThrown = 1;
     } catch (err) {
@@ -284,6 +288,7 @@ test(function headerParamsArgumentsCheck(): void {
     let errMsg = "";
 
     try {
+      // @ts-ignore
       headers[method]();
       hasThrown = 1;
     } catch (err) {
@@ -303,6 +308,7 @@ test(function headerParamsArgumentsCheck(): void {
     hasThrown = 0;
     errMsg = "";
     try {
+      // @ts-ignore
       headers[method]("foo");
       hasThrown = 1;
     } catch (err) {
diff --git a/cli/js/internals_test.ts b/cli/js/internals_test.ts
index 055995fdf..47aea09e7 100644
--- a/cli/js/internals_test.ts
+++ b/cli/js/internals_test.ts
@@ -4,7 +4,7 @@ import { test, assert } from "./test_util.ts";
 test(function internalsExists(): void {
   const {
     stringifyArgs
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-  } = Deno[Deno.symbols.internal] as any;
+    // @ts-ignore TypeScript (as of 3.7) does not support indexing namespaces by symbol
+  } = Deno[Deno.symbols.internal];
   assert(!!stringifyArgs);
 });
diff --git a/cli/js/mixins/dom_iterable_test.ts b/cli/js/mixins/dom_iterable_test.ts
index 466375d64..5dc45dc20 100644
--- a/cli/js/mixins/dom_iterable_test.ts
+++ b/cli/js/mixins/dom_iterable_test.ts
@@ -20,11 +20,8 @@ function setup() {
     Base,
     // This is using an internal API we don't want published as types, so having
     // to cast to any to "trick" TypeScript
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    DomIterable: (Deno[Deno.symbols.internal] as any).DomIterableMixin(
-      Base,
-      dataSymbol
-    )
+    // @ts-ignore TypeScript (as of 3.7) does not support indexing namespaces by symbol
+    DomIterable: Deno[Deno.symbols.internal].DomIterableMixin(Base, dataSymbol)
   };
 }
 
@@ -52,7 +49,12 @@ test(function testDomIterable(): void {
 
   result = [];
   const scope = {};
-  function callback(value, key, parent): void {
+  function callback(
+    this: typeof scope,
+    value: number,
+    key: string,
+    parent: typeof domIterable
+  ): void {
     assertEquals(parent, domIterable);
     assert(key != null);
     assert(value != null);
@@ -72,7 +74,7 @@ test(function testDomIterableScope(): void {
 
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   function checkScope(thisArg: any, expected: any): void {
-    function callback(): void {
+    function callback(this: typeof thisArg): void {
       assertEquals(this, expected);
     }
     domIterable.forEach(callback, thisArg);
diff --git a/cli/js/net_test.ts b/cli/js/net_test.ts
index e4d0be81f..68b1918b9 100644
--- a/cli/js/net_test.ts
+++ b/cli/js/net_test.ts
@@ -27,7 +27,7 @@ testPerm({ net: true }, async function netCloseWhileAccept(): Promise<void> {
 testPerm({ net: true }, async function netConcurrentAccept(): Promise<void> {
   const listener = Deno.listen({ port: 4502 });
   let acceptErrCount = 0;
-  const checkErr = (e): void => {
+  const checkErr = (e: Deno.DenoError<Deno.ErrorKind>): void => {
     assertEquals(e.kind, Deno.ErrorKind.Other);
     if (e.message === "Listener has been closed") {
       assertEquals(acceptErrCount, 1);
diff --git a/cli/js/os_test.ts b/cli/js/os_test.ts
index 6d9309000..a461ba63e 100644
--- a/cli/js/os_test.ts
+++ b/cli/js/os_test.ts
@@ -56,7 +56,10 @@ if (Deno.build.os === "win") {
     // specified in `inputEnv`. The subprocess reads the environment variables
     // which are in the keys of `expectedEnv` and writes them to stdout as JSON.
     // It is then verified that these match with the values of `expectedEnv`.
-    const checkChildEnv = async (inputEnv, expectedEnv): Promise<void> => {
+    const checkChildEnv = async (
+      inputEnv: Record<string, string>,
+      expectedEnv: Record<string, string>
+    ): Promise<void> => {
       const src = `
         console.log(
           ${JSON.stringify(Object.keys(expectedEnv))}.map(k => Deno.env(k))
@@ -249,6 +252,7 @@ testPerm({ env: true }, function getDir(): void {
       if (Deno.build.os !== r.os) continue;
       if (r.shouldHaveValue) {
         const d = Deno.dir(s.kind);
+        assert(d);
         assert(d.length > 0);
       }
     }
diff --git a/cli/js/process_test.ts b/cli/js/process_test.ts
index 943359e54..dce4d9918 100644
--- a/cli/js/process_test.ts
+++ b/cli/js/process_test.ts
@@ -130,6 +130,7 @@ testPerm({ run: true }, async function runStdinPiped(): Promise<void> {
     args: ["python", "-c", "import sys; assert 'hello' == sys.stdin.read();"],
     stdin: "piped"
   });
+  assert(p.stdin);
   assert(!p.stdout);
   assert(!p.stderr);
 
@@ -137,7 +138,7 @@ testPerm({ run: true }, async function runStdinPiped(): Promise<void> {
   const n = await p.stdin.write(msg);
   assertEquals(n, msg.byteLength);
 
-  p.stdin.close();
+  p.stdin!.close();
 
   const status = await p.status();
   assertEquals(status.success, true);
@@ -155,16 +156,16 @@ testPerm({ run: true }, async function runStdoutPiped(): Promise<void> {
   assert(!p.stderr);
 
   const data = new Uint8Array(10);
-  let r = await p.stdout.read(data);
+  let r = await p.stdout!.read(data);
   if (r === Deno.EOF) {
     throw new Error("p.stdout.read(...) should not be EOF");
   }
   assertEquals(r, 5);
   const s = new TextDecoder().decode(data.subarray(0, r));
   assertEquals(s, "hello");
-  r = await p.stdout.read(data);
+  r = await p.stdout!.read(data);
   assertEquals(r, Deno.EOF);
-  p.stdout.close();
+  p.stdout!.close();
 
   const status = await p.status();
   assertEquals(status.success, true);
@@ -182,16 +183,16 @@ testPerm({ run: true }, async function runStderrPiped(): Promise<void> {
   assert(!p.stdout);
 
   const data = new Uint8Array(10);
-  let r = await p.stderr.read(data);
+  let r = await p.stderr!.read(data);
   if (r === Deno.EOF) {
     throw new Error("p.stderr.read should not return EOF here");
   }
   assertEquals(r, 5);
   const s = new TextDecoder().decode(data.subarray(0, r));
   assertEquals(s, "hello");
-  r = await p.stderr.read(data);
+  r = await p.stderr!.read(data);
   assertEquals(r, Deno.EOF);
-  p.stderr.close();
+  p.stderr!.close();
 
   const status = await p.status();
   assertEquals(status.success, true);
@@ -307,7 +308,7 @@ testPerm({ run: true }, async function runClose(): Promise<void> {
   p.close();
 
   const data = new Uint8Array(10);
-  const r = await p.stderr.read(data);
+  const r = await p.stderr!.read(data);
   assertEquals(r, Deno.EOF);
 });
 
diff --git a/cli/js/resources_test.ts b/cli/js/resources_test.ts
index 367b66217..5d1e27af7 100644
--- a/cli/js/resources_test.ts
+++ b/cli/js/resources_test.ts
@@ -39,8 +39,8 @@ testPerm({ read: true }, async function resourcesFile(): Promise<void> {
     Object.keys(resourcesAfter).length,
     Object.keys(resourcesBefore).length + 1
   );
-  const newRid = Object.keys(resourcesAfter).find((rid): boolean => {
+  const newRid = +Object.keys(resourcesAfter).find((rid): boolean => {
     return !resourcesBefore.hasOwnProperty(rid);
-  });
+  })!;
   assertEquals(resourcesAfter[newRid], "fsFile");
 });
diff --git a/cli/js/signal_test.ts b/cli/js/signal_test.ts
index 06457314c..1c8658477 100644
--- a/cli/js/signal_test.ts
+++ b/cli/js/signal_test.ts
@@ -8,7 +8,7 @@ import {
 } from "./test_util.ts";
 
 function defer(n: number): Promise<void> {
-  return new Promise((resolve, _) => {
+  return new Promise((resolve: () => void, _) => {
     setTimeout(resolve, n);
   });
 }
diff --git a/cli/js/stat_test.ts b/cli/js/stat_test.ts
index 9b03c0ece..bce5449ac 100644
--- a/cli/js/stat_test.ts
+++ b/cli/js/stat_test.ts
@@ -210,7 +210,7 @@ if (isWindows) {
       const s = Deno.statSync(filename);
       assert(s.dev !== null);
       assert(s.ino !== null);
-      assertEquals(s.mode & 0o666, 0o666);
+      assertEquals(s.mode! & 0o666, 0o666);
       assertEquals(s.nlink, 2);
       assert(s.uid !== null);
       assert(s.gid !== null);
diff --git a/cli/js/test_util.ts b/cli/js/test_util.ts
index dbb7bf2c4..3e089486b 100644
--- a/cli/js/test_util.ts
+++ b/cli/js/test_util.ts
@@ -62,7 +62,10 @@ function permissionsMatch(
   requiredPerms: Permissions
 ): boolean {
   for (const permName in processPerms) {
-    if (processPerms[permName] !== requiredPerms[permName]) {
+    if (
+      processPerms[permName as keyof Permissions] !==
+      requiredPerms[permName as keyof Permissions]
+    ) {
       return false;
     }
   }
@@ -302,7 +305,7 @@ testPerm(
   async function assertAllUnitTestFilesImported(): Promise<void> {
     const directoryTestFiles = Deno.readDirSync("./cli/js")
       .map(k => k.name)
-      .filter(file => file.endsWith("_test.ts"));
+      .filter(file => file!.endsWith("_test.ts"));
     const unitTestsFile: Uint8Array = Deno.readFileSync(
       "./cli/js/unit_tests.ts"
     );
@@ -311,11 +314,11 @@ testPerm(
       .split("\n")
       .filter(line => line.startsWith("import") && line.includes("_test.ts"));
     const importedTestFiles = importLines.map(
-      relativeFilePath => relativeFilePath.match(/\/([^\/]+)";/)[1]
+      relativeFilePath => relativeFilePath.match(/\/([^\/]+)";/)![1]
     );
 
     directoryTestFiles.forEach(dirFile => {
-      if (!importedTestFiles.includes(dirFile)) {
+      if (!importedTestFiles.includes(dirFile!)) {
         throw new Error(
           "cil/js/unit_tests.ts is missing import of test file: cli/js/" +
             dirFile
diff --git a/cli/js/timers_test.ts b/cli/js/timers_test.ts
index b71df9254..84811ff11 100644
--- a/cli/js/timers_test.ts
+++ b/cli/js/timers_test.ts
@@ -7,21 +7,22 @@ function deferred(): {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   reject: (reason?: any) => void;
 } {
-  let resolve;
-  let reject;
-  const promise = new Promise((res, rej): void => {
+  let resolve: (value?: {} | PromiseLike<{}>) => void;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  let reject: ((reason?: any) => void) | undefined = undefined;
+  const promise = new Promise<{}>((res, rej): void => {
     resolve = res;
     reject = rej;
   });
   return {
     promise,
-    resolve,
-    reject
+    resolve: resolve!,
+    reject: reject!
   };
 }
 
-async function waitForMs(ms): Promise<number> {
-  return new Promise((resolve): number => setTimeout(resolve, ms));
+async function waitForMs(ms: number): Promise<number> {
+  return new Promise((resolve: () => void): number => setTimeout(resolve, ms));
 }
 
 test(async function timeoutSuccess(): Promise<void> {
@@ -133,7 +134,7 @@ test(async function intervalCancelSuccess(): Promise<void> {
 });
 
 test(async function intervalOrdering(): Promise<void> {
-  const timers = [];
+  const timers: number[] = [];
   let timeouts = 0;
   function onTimeout(): void {
     ++timeouts;
diff --git a/cli/js/tls_test.ts b/cli/js/tls_test.ts
index 1273da34f..ac59a2eb9 100644
--- a/cli/js/tls_test.ts
+++ b/cli/js/tls_test.ts
@@ -195,7 +195,7 @@ testPerm({ read: true, net: true }, async function dialAndListenTLS(): Promise<
   assertEquals(ok, "OK");
   const headers = await tpr.readMIMEHeader();
   assert(headers !== Deno.EOF);
-  const contentLength = parseInt(headers.get("content-length"));
+  const contentLength = parseInt(headers.get("content-length")!);
   const bodyBuf = new Uint8Array(contentLength);
   await r.readFull(bodyBuf);
   assertEquals(decoder.decode(bodyBuf), "Hello World\n");
diff --git a/cli/js/url_search_params_test.ts b/cli/js/url_search_params_test.ts
index c1343e59b..8619568bd 100644
--- a/cli/js/url_search_params_test.ts
+++ b/cli/js/url_search_params_test.ts
@@ -177,6 +177,7 @@ test(function urlSearchParamsAppendArgumentsCheck(): void {
       const searchParams = new URLSearchParams();
       let hasThrown = 0;
       try {
+        // @ts-ignore
         searchParams[method]();
         hasThrown = 1;
       } catch (err) {
@@ -193,6 +194,7 @@ test(function urlSearchParamsAppendArgumentsCheck(): void {
     const searchParams = new URLSearchParams();
     let hasThrown = 0;
     try {
+      // @ts-ignore
       searchParams[method]("foo");
       hasThrown = 1;
     } catch (err) {
@@ -232,6 +234,7 @@ test(function urlSearchParamsCustomSymbolIterator(): void {
 
 test(function urlSearchParamsCustomSymbolIteratorWithNonStringParams(): void {
   const params = {};
+  // @ts-ignore
   params[Symbol.iterator] = function*(): IterableIterator<[number, number]> {
     yield [1, 2];
   };
diff --git a/cli/js/utime_test.ts b/cli/js/utime_test.ts
index 15c218df4..72a4a6477 100644
--- a/cli/js/utime_test.ts
+++ b/cli/js/utime_test.ts
@@ -3,7 +3,9 @@ import { testPerm, assert, assertEquals } from "./test_util.ts";
 
 // Allow 10 second difference.
 // Note this might not be enough for FAT (but we are not testing on such fs).
-function assertFuzzyTimestampEquals(t1: number, t2: number): void {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function assertFuzzyTimestampEquals(t1: any, t2: number): void {
+  assert(typeof t1 === "number");
   assert(Math.abs(t1 - t2) < 10);
 }
 
diff --git a/cli/js/write_file_test.ts b/cli/js/write_file_test.ts
index 30500b594..2b952655f 100644
--- a/cli/js/write_file_test.ts
+++ b/cli/js/write_file_test.ts
@@ -50,9 +50,9 @@ testPerm({ read: true, write: true }, function writeFileSyncUpdatePerm(): void {
     const data = enc.encode("Hello");
     const filename = Deno.makeTempDirSync() + "/test.txt";
     Deno.writeFileSync(filename, data, { perm: 0o755 });
-    assertEquals(Deno.statSync(filename).mode & 0o777, 0o755);
+    assertEquals(Deno.statSync(filename).mode! & 0o777, 0o755);
     Deno.writeFileSync(filename, data, { perm: 0o666 });
-    assertEquals(Deno.statSync(filename).mode & 0o777, 0o666);
+    assertEquals(Deno.statSync(filename).mode! & 0o777, 0o666);
   }
 });
 
@@ -161,9 +161,9 @@ testPerm(
       const data = enc.encode("Hello");
       const filename = Deno.makeTempDirSync() + "/test.txt";
       await Deno.writeFile(filename, data, { perm: 0o755 });
-      assertEquals(Deno.statSync(filename).mode & 0o777, 0o755);
+      assertEquals(Deno.statSync(filename).mode! & 0o777, 0o755);
       await Deno.writeFile(filename, data, { perm: 0o666 });
-      assertEquals(Deno.statSync(filename).mode & 0o777, 0o666);
+      assertEquals(Deno.statSync(filename).mode! & 0o777, 0o666);
     }
   }
 );