diff --git a/.eslintrc.json b/.eslintrc.json
index 1f39c29101f83..b10c839600b95 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -49,6 +49,7 @@
             "allowDeclarations": true
         }],
         "no-double-space": "error",
+        "async-type": "error",
         "boolean-trivia": "error",
         "no-in-operator": "error",
         "simple-indent": "error",
diff --git a/scripts/eslint/rules/async-type.ts b/scripts/eslint/rules/async-type.ts
new file mode 100644
index 0000000000000..7e619445bbe4a
--- /dev/null
+++ b/scripts/eslint/rules/async-type.ts
@@ -0,0 +1,40 @@
+import { AST_NODE_TYPES, TSESTree } from "@typescript-eslint/experimental-utils";
+import { createRule } from "./utils";
+
+export = createRule({
+    name: "async-type",
+    meta: {
+        docs: {
+            description: ``,
+            category: "Possible Errors",
+            recommended: "error",
+        },
+        messages: {
+            asyncTypeError: `Async functions must have an explicit return type`,
+        },
+        schema: [],
+        type: "problem",
+    },
+    defaultOptions: [],
+
+    create(context) {
+        const checkAsyncFunction = (node: TSESTree.FunctionDeclaration | TSESTree.FunctionExpression | TSESTree.ArrowFunctionExpression) => {
+            if (node.async && !node.returnType) {
+                context.report({ messageId: "asyncTypeError", node });
+            }
+        };
+
+        const checkAsyncMethod = (node: TSESTree.MethodDefinition) => {
+            if (node.value.type === AST_NODE_TYPES.FunctionExpression) {
+                checkAsyncFunction(node.value);
+            }
+        };
+
+        return {
+            [AST_NODE_TYPES.FunctionDeclaration]: checkAsyncFunction,
+            [AST_NODE_TYPES.FunctionExpression]: checkAsyncFunction,
+            [AST_NODE_TYPES.ArrowFunctionExpression]: checkAsyncFunction,
+            [AST_NODE_TYPES.MethodDefinition]: checkAsyncMethod
+        };
+    },
+});
diff --git a/scripts/eslint/tests/async-type.test.ts b/scripts/eslint/tests/async-type.test.ts
new file mode 100644
index 0000000000000..b80bbae7b4452
--- /dev/null
+++ b/scripts/eslint/tests/async-type.test.ts
@@ -0,0 +1,55 @@
+import { RuleTester } from "./support/RuleTester";
+import rule = require("../rules/async-type");
+
+const ruleTester = new RuleTester({
+    parserOptions: {
+        warnOnUnsupportedTypeScriptVersion: false,
+    },
+    parser: require.resolve("@typescript-eslint/parser"),
+});
+
+ruleTester.run("async-type", rule, {
+    valid: [
+        {
+            code: `
+async function foo(): Promise<void> {}
+            `,
+        },
+        {
+            code: `
+const fn = async function(): Promise<void> {}
+            `,
+        },
+        {
+            code: `
+const fn = async (): Promise<void> => {}
+            `,
+        },
+        {
+            code: `
+class C {
+    async method(): Promise<void> {}
+}
+            `,
+        },
+    ],
+
+    invalid: [
+        {
+            code: `async function foo() {}`,
+            errors: [{ messageId: "asyncTypeError" }]
+        },
+        {
+            code: `const fn = async function() {}`,
+            errors: [{ messageId: "asyncTypeError" }]
+        },
+        {
+            code: `const fn = async () => {}`,
+            errors: [{ messageId: "asyncTypeError" }]
+        },
+        {
+            code: `class C { async method() {} }`,
+            errors: [{ messageId: "asyncTypeError" }]
+        },
+    ]
+});
\ No newline at end of file
diff --git a/scripts/open-cherry-pick-pr.ts b/scripts/open-cherry-pick-pr.ts
index ac39d959bd973..e8ae3c9a44c5f 100644
--- a/scripts/open-cherry-pick-pr.ts
+++ b/scripts/open-cherry-pick-pr.ts
@@ -1,7 +1,9 @@
+/* eslint-disable async-type */
 /// <reference lib="esnext.asynciterable" />
 // Must reference esnext.asynciterable lib, since octokit uses AsyncIterable internally
 /// <reference types="node" />
 
+
 import Octokit = require("@octokit/rest");
 const {runSequence} = require("./run-sequence");
 import fs = require("fs");
diff --git a/scripts/open-user-pr.ts b/scripts/open-user-pr.ts
index a2472f3a9fab7..2b1597f736c60 100644
--- a/scripts/open-user-pr.ts
+++ b/scripts/open-user-pr.ts
@@ -1,5 +1,7 @@
+/* eslint-disable async-type */
 /// <reference lib="esnext.asynciterable" />
 /// <reference lib="es2015.promise" />
+
 // Must reference esnext.asynciterable lib, since octokit uses AsyncIterable internally
 import Octokit = require("@octokit/rest");
 import {runSequence} from "./run-sequence";
diff --git a/scripts/produceLKG.ts b/scripts/produceLKG.ts
index 762290bcfa6a0..f4da4e8fc6fdb 100644
--- a/scripts/produceLKG.ts
+++ b/scripts/produceLKG.ts
@@ -1,3 +1,4 @@
+/* eslint-disable async-type */
 /// <reference types="node" />
 
 import childProcess = require("child_process");
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 4941257cabf64..e350c01d0d20a 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -58,6 +58,23 @@ namespace ts {
         push(...values: T[]): void;
     }
 
+    /* @internal */
+    export interface PromiseConstructor extends PromiseConstructorLike {
+        new <T>(executor: (resolve: (value: T | PromiseLike<T>) => void, reject: (reason: unknown) => void) => void): Promise<T>;
+        prototype: Promise<any>;
+        resolve<T>(value: T | PromiseLike<T>): Promise<T>;
+        resolve(): Promise<void>;
+        reject<T = never>(reason: unknown): Promise<T>;
+        all<T>(promises: (T | PromiseLike<T>)[]): Promise<T[]>;
+        race<T>(promises: (T | PromiseLike<T>)[]): Promise<T>;
+    }
+
+    /* @internal */
+    export interface Promise<T> {
+        then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: unknown) => TResult2 | PromiseLike<TResult2>) | undefined | null): Promise<TResult1 | TResult2>;
+        catch<TResult = never>(onrejected?: ((reason: unknown) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult>;
+    }
+
     /* @internal */
     export type EqualityComparer<T> = (a: T, b: T) => boolean;
 
@@ -75,8 +92,9 @@ namespace ts {
 /* @internal */
 namespace ts {
     // Natives
-    // NOTE: This must be declared in a separate block from the one below so that we don't collide with the exported definition of `Map`.
-    declare const Map: (new <T>() => Map<T>) | undefined;
+    // NOTE: These must be declared in a separate block from the one below so that we don't collide with the exported definitions of `Map` and `Promise`.
+    declare const Map: MapConstructor | undefined;
+    declare const Promise: PromiseConstructor | undefined;
 
     /**
      * Returns the native Map implementation if it is available and compatible (i.e. supports iteration).
@@ -86,6 +104,10 @@ namespace ts {
         // eslint-disable-next-line no-in-operator
         return typeof Map !== "undefined" && "entries" in Map.prototype ? Map : undefined;
     }
+
+    export function tryGetNativePromise(): PromiseConstructor | undefined {
+        return typeof Promise === "function" ? Promise : undefined;
+    }
 }
 
 /* @internal */
@@ -100,6 +122,14 @@ namespace ts {
         throw new Error("TypeScript requires an environment that provides a compatible native Map implementation.");
     })();
 
+    export const Promise: PromiseConstructor = tryGetNativePromise() || (() => {
+        // NOTE: createPromiseShim will be defined for typescriptServices.js but not for tsc.js, so we must test for it.
+        if (typeof createPromiseShim === "function") {
+            return createPromiseShim();
+        }
+        throw new Error("TypeScript requires an environment that provides a compatible native Promise implementation.");
+    })();
+
     /** Create a new map. */
     export function createMap<T>(): Map<T> {
         return new Map<T>();
diff --git a/src/shims/promiseShim.ts b/src/shims/promiseShim.ts
new file mode 100644
index 0000000000000..448d0dc331528
--- /dev/null
+++ b/src/shims/promiseShim.ts
@@ -0,0 +1,172 @@
+/* @internal */
+namespace ts {
+    declare function setTimeout<A extends any[]>(handler: (...args: A) => void, timeout: number, ...args: A): any;
+
+    export interface Promise<T> extends globalThis.Promise<T> {
+    }
+
+    export interface PromiseConstructor extends PromiseConstructorLike {
+        new <T>(executor: (resolve: (value: T | PromiseLike<T>) => void, reject: (reason: unknown) => void) => void): Promise<T>;
+        resolve<T>(value: T | PromiseLike<T>): Promise<T>;
+        resolve(): Promise<void>;
+        reject<T = never>(reason: unknown): Promise<T>;
+        all<T>(promises: (T | PromiseLike<T>)[]): Promise<T[]>;
+        race<T>(promises: (T | PromiseLike<T>)[]): Promise<T>;
+    }
+
+    export function createPromiseShim(): PromiseConstructor {
+        class Promise<T> implements ts.Promise<T> {
+            private _state: "pending" | "fulfilled" | "rejected" = "pending";
+            private _result: unknown;
+            private _reactions: PromiseReaction[] = [];
+
+            constructor(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason: any) => void) => void) {
+                const { resolve, reject } = createResolvingFunctions(this);
+                try {
+                    executor(resolve, reject);
+                }
+                catch (e) {
+                    reject(e);
+                }
+            }
+
+            then<TResult1 = T, TResult2 = never>(
+                onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null,
+                onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null
+            ): Promise<TResult1 | TResult2> {
+                return new Promise<TResult1 | TResult2>((resolve, reject) => {
+                    const reaction: PromiseReaction = { resolve, reject, onfulfilled, onrejected };
+                    if (this._state === "pending") {
+                        this._reactions.push(reaction);
+                    }
+                    else {
+                        setTimeout(promiseReactionJob, 0, reaction, this._state === "fulfilled" ? "fulfill" : "reject", this._result);
+                    }
+                });
+            }
+
+            catch<TResult = never>(onrejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): Promise<T | TResult> {
+                return this.then(/*onfulfilled*/ undefined, onrejected);
+            }
+
+            static resolve(): Promise<void>;
+            static resolve<T>(value: T | PromiseLike<T>): Promise<T>;
+            static resolve<T>(value?: T | PromiseLike<T>): Promise<T> {
+                return value instanceof this && value.constructor === this ? value : new Promise<T>(resolve => resolve(value));
+            }
+
+            static reject<T = never>(reason: any): Promise<T> {
+                return new Promise<T>((_, reject) => reject(reason));
+            }
+
+            static all<T>(promises: (T | PromiseLike<T>)[]): Promise<T[]> {
+                return new Promise<T[]>((resolve, reject) => {
+                    let count = promises.length;
+                    const values: T[] = Array<T>(count);
+                    for (let i = 0; i < promises.length; i++) {
+                        let called = false;
+                        this.resolve(promises[i]).then(
+                            value => {
+                                if (!called) {
+                                    called = true;
+                                    values[i] = value;
+                                    count--;
+                                    if (count === 0) {
+                                        resolve(values);
+                                    }
+                                }
+                            },
+                            reject);
+                    }
+                });
+            }
+
+            static race<T>(promises: (T | PromiseLike<T>)[]): Promise<T> {
+                return new Promise<T>((resolve, reject) => {
+                    for (const promise of promises) {
+                        this.resolve(promise).then(resolve, reject);
+                    }
+                });
+            }
+        }
+
+        interface PromiseReaction {
+            resolve: (value: unknown) => void;
+            reject: (reason: unknown) => void;
+            onfulfilled?: ((value: unknown) => unknown) | null;
+            onrejected?: ((reason: unknown) => unknown) | null;
+        }
+
+        function createResolvingFunctions<T>(promise: Promise<T>) {
+            let called = false;
+            return {
+                resolve: (value: T | Promise<T>) => {
+                    if (!called) {
+                        called = true;
+                        try {
+                            if (promise === value) throw new TypeError();
+                            // eslint-disable-next-line no-null/no-null
+                            const then = typeof value === "object" && value !== null && (<Promise<unknown>>value).then;
+                            if (typeof then !== "function") {
+                                settlePromise(promise, "fulfill", value);
+                            }
+                            else {
+                                setTimeout(resolveThenableJob, 0, promise, value, then);
+                            }
+                        }
+                        catch (e) {
+                            settlePromise(promise, "reject", e);
+                        }
+                    }
+                },
+                reject: (reason: any) => {
+                    if (!called) {
+                        called = true;
+                        settlePromise(promise, "reject", reason);
+                    }
+                }
+            };
+        }
+
+        function settlePromise(promise: Promise<unknown>, verb: "fulfill" | "reject", value: unknown) {
+            /* eslint-disable dot-notation */
+            const reactions = promise["_reactions"];
+            promise["_result"] = value;
+            promise["_reactions"] = undefined!;
+            promise["_state"] = verb === "fulfill" ? "fulfilled" : "rejected";
+            for (const reaction of reactions) {
+                setTimeout(promiseReactionJob, 0, reaction, verb, value);
+            }
+            /* eslint-enable dot-notation */
+        }
+
+        function resolveThenableJob<T>(promiseToResolve: Promise<T>, thenable: T, thenAction: Promise<T>["then"]) {
+            const { resolve, reject } = createResolvingFunctions(promiseToResolve);
+            try {
+                thenAction.call(thenable, resolve, reject);
+            }
+            catch (e) {
+                reject(e);
+            }
+        }
+
+        function promiseReactionJob(reaction: PromiseReaction, verb: "fulfill" | "reject", argument: unknown) {
+            const handler = verb === "fulfill" ? reaction.onfulfilled : reaction.onrejected;
+            if (handler) {
+                try {
+                    argument = handler(argument);
+                    verb = "fulfill";
+                }
+                catch (e) {
+                    argument = e;
+                    verb = "reject";
+                }
+            }
+
+            const action = verb === "fulfill" ? reaction.resolve : reaction.reject;
+            action(argument);
+        }
+
+        return Promise;
+    }
+}
\ No newline at end of file
diff --git a/src/shims/tsconfig.json b/src/shims/tsconfig.json
index cb058762fd616..d433975cf9d3d 100644
--- a/src/shims/tsconfig.json
+++ b/src/shims/tsconfig.json
@@ -4,6 +4,7 @@
         "outFile": "../../built/local/shims.js"
     },
     "files": [
-        "mapShim.ts"
+        "mapShim.ts",
+        "promiseShim.ts"
     ]
 }
diff --git a/src/testRunner/tsconfig.json b/src/testRunner/tsconfig.json
index 27a7712a2b14f..027154abd65cf 100644
--- a/src/testRunner/tsconfig.json
+++ b/src/testRunner/tsconfig.json
@@ -62,6 +62,7 @@
         "unittests/reuseProgramStructure.ts",
         "unittests/semver.ts",
         "unittests/createMapShim.ts",
+        "unittests/createPromiseShim.ts",
         "unittests/transform.ts",
         "unittests/config/commandLineParsing.ts",
         "unittests/config/configurationExtension.ts",
diff --git a/src/testRunner/unittests/createPromiseShim.ts b/src/testRunner/unittests/createPromiseShim.ts
new file mode 100644
index 0000000000000..34b84b61f890b
--- /dev/null
+++ b/src/testRunner/unittests/createPromiseShim.ts
@@ -0,0 +1,131 @@
+describe("unittests:: createPromiseShim", () => {
+    function expectFulfilled<T>(done: Mocha.Done, value: T) {
+        return (_value: T) => {
+            try {
+                assert.strictEqual(_value, value);
+            }
+            catch (e) {
+                return done(e);
+            }
+            done();
+        };
+    }
+
+    function expectNotFulfilled(done: Mocha.Done) {
+        return () => done(new Error("expected promise to be rejected"));
+    }
+
+    function expectNotRejected(done: Mocha.Done) {
+        return (_reason: any) => done(_reason);
+    }
+
+    function expectRejected(done: Mocha.Done, reason: any) {
+        return (_reason: any) => {
+            try {
+                assert.strictEqual(_reason, reason);
+            }
+            catch (e) {
+                return done(e);
+            }
+            done();
+        };
+    }
+
+    it("executor resolves correctly with non-promise", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = {};
+        new PromiseShim(resolve => resolve(value)).then(
+            expectFulfilled(done, value),
+            expectNotRejected(done)
+        );
+    }).timeout(500);
+
+    it("executor resolves correctly with promise", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = { };
+        const p = { then(cb: (v: {}) => void) { cb(value); } };
+        new PromiseShim(resolve => resolve(p)).then(
+            expectFulfilled(done, value),
+            expectNotRejected(done)
+        );
+    }).timeout(500);
+
+    it("executor rejects correctly", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = { };
+        new PromiseShim((_, reject) => reject(value)).then(
+            expectNotFulfilled(done),
+            expectRejected(done, value)
+        );
+    }).timeout(500);
+
+    it("catches error in executor", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = {};
+        new PromiseShim(() => { throw value; }).then(
+            expectNotFulfilled(done),
+            expectRejected(done, value)
+        );
+    }).timeout(500);
+
+    it("catches error in onfulfilled of 'then'", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = {};
+        new PromiseShim(resolve => resolve(/*value*/ undefined)).then(
+            () => { throw value; },
+            expectNotRejected(done)
+        ).then(
+            expectNotFulfilled(done),
+            expectRejected(done, value)
+        );
+    }).timeout(500);
+
+    it("catches error in onrejected of 'then'", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const first = {};
+        const second = {};
+        new PromiseShim((_, reject) => reject(first)).then(
+            expectNotFulfilled(done),
+            _ => { throw second; }
+        ).then(
+            expectNotFulfilled(done),
+            expectRejected(done, second)
+        );
+    }).timeout(500);
+
+    it("Promise.resolve resolves correctly with non-promise", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = {};
+        PromiseShim.resolve(value).then(
+            expectFulfilled(done, value),
+            expectNotRejected(done)
+        );
+    }).timeout(500);
+
+    it("Promsie.resolve resolves correctly with promise", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = { };
+        const p = { then(cb: (v: {}) => void) { cb(value); } };
+        PromiseShim.resolve(p).then(
+            expectFulfilled(done, value),
+            expectNotRejected(done)
+        );
+    }).timeout(500);
+
+    it("Promsie.resolve returns same promise if instance", () => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = { };
+        const first = PromiseShim.resolve(value);
+        const second = PromiseShim.resolve(first);
+        assert.strictEqual(first, second);
+    });
+
+    it("Promise.reject rejects correctly", done => {
+        const PromiseShim = ts.createPromiseShim(); // tslint:disable-line variable-name
+        const value = { };
+        PromiseShim.reject(value).then(
+            expectNotFulfilled(done),
+            expectRejected(done, value)
+        );
+    }).timeout(500);
+});
\ No newline at end of file
diff --git a/src/testRunner/unittests/evaluation/asyncArrow.ts b/src/testRunner/unittests/evaluation/asyncArrow.ts
index 04a7af613fa38..0effd59e08274 100644
--- a/src/testRunner/unittests/evaluation/asyncArrow.ts
+++ b/src/testRunner/unittests/evaluation/asyncArrow.ts
@@ -1,6 +1,6 @@
 describe("unittests:: evaluation:: asyncArrowEvaluation", () => {
     // https://github.com/Microsoft/TypeScript/issues/24722
-    it("this capture (es5)", async () => {
+    it("this capture (es5)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         export class A {
             b = async (...args: any[]) => {
diff --git a/src/testRunner/unittests/evaluation/asyncGenerator.ts b/src/testRunner/unittests/evaluation/asyncGenerator.ts
index 8ca531f0759a1..afdefec243c4f 100644
--- a/src/testRunner/unittests/evaluation/asyncGenerator.ts
+++ b/src/testRunner/unittests/evaluation/asyncGenerator.ts
@@ -1,5 +1,5 @@
 describe("unittests:: evaluation:: asyncGeneratorEvaluation", () => {
-    it("return (es5)", async () => {
+    it("return (es5)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         async function * g() {
             return Promise.resolve(0);
@@ -13,7 +13,7 @@ describe("unittests:: evaluation:: asyncGeneratorEvaluation", () => {
             { value: 0, done: true }
         ]);
     });
-    it("return (es2015)", async () => {
+    it("return (es2015)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         async function * g() {
             return Promise.resolve(0);
diff --git a/src/testRunner/unittests/evaluation/awaiter.ts b/src/testRunner/unittests/evaluation/awaiter.ts
index 65cb4e0ead945..4ad15dc11db39 100644
--- a/src/testRunner/unittests/evaluation/awaiter.ts
+++ b/src/testRunner/unittests/evaluation/awaiter.ts
@@ -1,6 +1,6 @@
 describe("unittests:: evaluation:: awaiter", () => {
     // NOTE: This could break if the ECMAScript spec ever changes the timing behavior for Promises (again)
-    it("await (es5)", async () => {
+    it("await (es5)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         async function a(msg: string) {
             await Promise.resolve();
diff --git a/src/testRunner/unittests/evaluation/forAwaitOf.ts b/src/testRunner/unittests/evaluation/forAwaitOf.ts
index c7be018cbc9e6..a973fa74b951b 100644
--- a/src/testRunner/unittests/evaluation/forAwaitOf.ts
+++ b/src/testRunner/unittests/evaluation/forAwaitOf.ts
@@ -1,5 +1,5 @@
 describe("unittests:: evaluation:: forAwaitOfEvaluation", () => {
-    it("sync (es5)", async () => {
+    it("sync (es5)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         let i = 0;
         const iterator: IterableIterator<any> = {
@@ -25,7 +25,7 @@ describe("unittests:: evaluation:: forAwaitOfEvaluation", () => {
         assert.strictEqual(result.output[2], 3);
     });
 
-    it("sync (es2015)", async () => {
+    it("sync (es2015)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         let i = 0;
         const iterator: IterableIterator<any> = {
@@ -51,7 +51,7 @@ describe("unittests:: evaluation:: forAwaitOfEvaluation", () => {
         assert.strictEqual(result.output[2], 3);
     });
 
-    it("async (es5)", async () => {
+    it("async (es5)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         let i = 0;
         const iterator = {
@@ -77,7 +77,7 @@ describe("unittests:: evaluation:: forAwaitOfEvaluation", () => {
         assert.instanceOf(result.output[2], Promise);
     });
 
-    it("async (es2015)", async () => {
+    it("async (es2015)", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
         let i = 0;
         const iterator = {
diff --git a/src/testRunner/unittests/evaluation/objectRest.ts b/src/testRunner/unittests/evaluation/objectRest.ts
index 272ba51ffbeb5..56f0180d88913 100644
--- a/src/testRunner/unittests/evaluation/objectRest.ts
+++ b/src/testRunner/unittests/evaluation/objectRest.ts
@@ -1,6 +1,6 @@
 describe("unittests:: evaluation:: objectRest", () => {
     // https://github.com/microsoft/TypeScript/issues/31469
-    it("side effects in property assignment", async () => {
+    it("side effects in property assignment", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             const k = { a: 1, b: 2 };
             const o = { a: 3, ...k, b: k.a++ };
@@ -8,7 +8,7 @@ describe("unittests:: evaluation:: objectRest", () => {
         `);
         assert.deepEqual(result.output, { a: 1, b: 1 });
     });
-    it("side effects in during spread", async () => {
+    it("side effects in during spread", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             const k = { a: 1, get b() { l = { c: 9 }; return 2; } };
             let l = { c: 3 };
@@ -17,7 +17,7 @@ describe("unittests:: evaluation:: objectRest", () => {
         `);
         assert.deepEqual(result.output, { a: 1, b: 2, c: 9 });
     });
-    it("trailing literal-valued object-literal", async () => {
+    it("trailing literal-valued object-literal", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             const k = { a: 1 }
             const o = { ...k, ...{ b: 2 } };
diff --git a/src/testRunner/unittests/evaluation/optionalCall.ts b/src/testRunner/unittests/evaluation/optionalCall.ts
index d6317ad0a8ca9..c8163bff4d73b 100644
--- a/src/testRunner/unittests/evaluation/optionalCall.ts
+++ b/src/testRunner/unittests/evaluation/optionalCall.ts
@@ -1,5 +1,5 @@
 describe("unittests:: evaluation:: optionalCall", () => {
-    it("f?.()", async () => {
+    it("f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             function f(a) {
                 output.push(a);
@@ -11,7 +11,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.isUndefined(result.output[1]);
     });
-    it("o.f?.()", async () => {
+    it("o.f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 f(a) {
@@ -25,7 +25,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o);
     });
-    it("o.x.f?.()", async () => {
+    it("o.x.f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 x: {
@@ -41,7 +41,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o.x);
     });
-    it("o?.f()", async () => {
+    it("o?.f()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 f(a) {
@@ -55,7 +55,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o);
     });
-    it("o?.f?.()", async () => {
+    it("o?.f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 f(a) {
@@ -69,7 +69,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o);
     });
-    it("o.x?.f()", async () => {
+    it("o.x?.f()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 x: {
@@ -85,7 +85,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o.x);
     });
-    it("o?.x.f()", async () => {
+    it("o?.x.f()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 x: {
@@ -101,7 +101,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o.x);
     });
-    it("o?.x?.f()", async () => {
+    it("o?.x?.f()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 x: {
@@ -117,7 +117,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o.x);
     });
-    it("o?.x?.f?.()", async () => {
+    it("o?.x?.f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 x: {
@@ -133,7 +133,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[0], 1);
         assert.strictEqual(result.output[1], result.o.x);
     });
-    it("f?.()?.()", async () => {
+    it("f?.()?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             function g(a) {
                 output.push(a);
@@ -150,7 +150,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[1], 2);
         assert.isUndefined(result.output[2]);
     });
-    it("f?.().f?.()", async () => {
+    it("f?.().f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 f(a) {
@@ -169,7 +169,7 @@ describe("unittests:: evaluation:: optionalCall", () => {
         assert.strictEqual(result.output[1], 2);
         assert.strictEqual(result.output[2], result.o);
     });
-    it("f?.()?.f?.()", async () => {
+    it("f?.()?.f?.()", async (): Promise<void> => {
         const result = evaluator.evaluateTypeScript(`
             export const o = {
                 f(a) {