diff --git a/.eslintignore b/.eslintignore
new file mode 100644
index 0000000000..9f113730f7
--- /dev/null
+++ b/.eslintignore
@@ -0,0 +1,4 @@
+/flags/
+/fs/
+/http/
+/ws/
diff --git a/.eslintrc.json b/.eslintrc.json
new file mode 100644
index 0000000000..43405e21eb
--- /dev/null
+++ b/.eslintrc.json
@@ -0,0 +1,23 @@
+{
+  "root": true,
+  "parser": "@typescript-eslint/parser",
+  "parserOptions": {
+    "project": "./tsconfig.json"
+  },
+  "plugins": ["@typescript-eslint"],
+  "extends": [
+    "plugin:@typescript-eslint/recommended",
+    "prettier",
+    "prettier/@typescript-eslint"
+  ],
+  "rules": {
+    "@typescript-eslint/array-type": ["error", "array-simple"],
+    "@typescript-eslint/explicit-member-accessibility": ["off"],
+    "@typescript-eslint/no-non-null-assertion": ["off"],
+    "@typescript-eslint/no-parameter-properties": ["off"],
+    "@typescript-eslint/no-unused-vars": [
+      "error",
+      { "argsIgnorePattern": "^_" }
+    ]
+  }
+}
diff --git a/.gitignore b/.gitignore
index b2941c3c2b..bb7de9d001 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 .DS_Store
 .idea
-tsconfig.json
-deno.d.ts
\ No newline at end of file
+deno.d.ts
+node_modules
+package.json
+package-lock.json
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 2d6f318a4c..861159e845 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -1,31 +1,39 @@
 variables:
-  DENO_VERSION: 'v0.3.1'
+  DENO_VERSION: "v0.3.1"
+  TS_VERSION: "3.2.1"
 
+# TODO DRY up the jobs
+# TODO Try to get eslint to run under Deno, like prettier
 jobs:
+  - job: "Linux"
+    pool:
+      vmImage: "Ubuntu-16.04"
+    steps:
+      - script: npm install eslint typescript@$(TS_VERSION) @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-config-prettier
+      - script: curl -L https://deno.land/x/install/install.sh | sh -s $(DENO_VERSION)
+      - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
+      - script: npx eslint **/*.ts
+      - script: deno format.ts --allow-run --allow-write --allow-read --check
+      - script: deno test.ts --allow-run --allow-net --allow-write --allow-read
 
-- job: 'Linux'
-  pool:
-    vmImage: 'Ubuntu-16.04'
-  steps:
-  - script: curl -L https://deno.land/x/install/install.sh | sh -s $(DENO_VERSION)
-  - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
-  - script: deno test.ts --allow-run --allow-net --allow-write --allow-read
-  - script: deno format.ts --allow-run --allow-write --allow-read --check
+  - job: "Mac"
+    pool:
+      vmImage: "macOS-10.13"
+    steps:
+      - script: npm -g install eslint typescript@$(TS_VERSION) @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-config-prettier
+      - script: curl -L https://deno.land/x/install/install.sh | sh -s $(DENO_VERSION)
+      - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
+      - script: eslint **/*.ts
+      - script: deno format.ts --allow-run --allow-write --allow-read --check
+      - script: deno test.ts --allow-run --allow-net --allow-write --allow-read
 
-- job: 'Mac'
-  pool:
-    vmImage: 'macOS-10.13'
-  steps:
-  - script: curl -L https://deno.land/x/install/install.sh | sh -s $(DENO_VERSION)
-  - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
-  - script: deno test.ts --allow-run --allow-net --allow-write --allow-read
-  - script: deno format.ts --allow-run --allow-write --allow-read --check
-
-- job: 'Windows'
-  pool:
-    vmImage: 'vs2017-win2016'
-  steps:
-  - powershell: iwr https://deno.land/x/install/install.ps1 -out install.ps1; .\install.ps1 $(DENO_VERSION)
-  - bash: echo "##vso[task.prependpath]C:\Users\VssAdministrator\.deno\\bin"
-  - bash: deno.exe test.ts --allow-run --allow-net --allow-write --allow-read
-  - bash: deno.exe format.ts --allow-run --allow-write --allow-read --check
+  - job: "Windows"
+    pool:
+      vmImage: "vs2017-win2016"
+    steps:
+      - bash: npm install eslint typescript@$(TS_VERSION) @typescript-eslint/eslint-plugin @typescript-eslint/parser eslint-config-prettier
+      - powershell: iwr https://deno.land/x/install/install.ps1 -out install.ps1; .\install.ps1 $(DENO_VERSION)
+      - bash: echo "##vso[task.prependpath]C:\Users\VssAdministrator\.deno\\bin"
+      - bash: npx eslint **/*.ts
+      - bash: deno.exe format.ts --allow-run --allow-write --allow-read --check
+      - bash: deno.exe test.ts --allow-run --allow-net --allow-write --allow-read
diff --git a/benching/example.ts b/benching/example.ts
index 1018dd9e1a..1563379bfc 100644
--- a/benching/example.ts
+++ b/benching/example.ts
@@ -14,7 +14,7 @@ bench({
   runs: 100,
   func(b: BenchmarkTimer) {
     b.start();
-    for (let i: number = 0; i < 1e6; i++);
+    for (let i = 0; i < 1e6; i++);
     b.stop();
   }
 });
diff --git a/benching/mod.ts b/benching/mod.ts
index 4d2f5d41ec..6fa442d032 100644
--- a/benching/mod.ts
+++ b/benching/mod.ts
@@ -14,10 +14,10 @@ export interface BenchmarkTimer {
 }
 
 /** Defines a benchmark through a named function. */
-export type BenchmarkFunction = {
+export interface BenchmarkFunction {
   (b: BenchmarkTimer): void | Promise<void>;
   name: string;
-};
+}
 
 /** Defines a benchmark definition with configurable runs. */
 export interface BenchmarkDefinition {
@@ -69,7 +69,7 @@ function createBenchmarkTimer(clock: BenchmarkClock): BenchmarkTimer {
   };
 }
 
-const candidates: Array<BenchmarkDefinition> = [];
+const candidates: BenchmarkDefinition[] = [];
 
 /** Registers a benchmark as a candidate for the runBenchmarks executor. */
 export function bench(
@@ -95,27 +95,27 @@ export async function runBenchmarks({
   skip = /^\s*$/
 }: BenchmarkRunOptions = {}): Promise<void> {
   // Filtering candidates by the "only" and "skip" constraint
-  const benchmarks: Array<BenchmarkDefinition> = candidates.filter(
+  const benchmarks: BenchmarkDefinition[] = candidates.filter(
     ({ name }) => only.test(name) && !skip.test(name)
   );
   // Init main counters and error flag
-  const filtered: number = candidates.length - benchmarks.length;
-  let measured: number = 0;
-  let failed: boolean = false;
+  const filtered = candidates.length - benchmarks.length;
+  let measured = 0;
+  let failed = false;
   // Setting up a shared benchmark clock and timer
   const clock: BenchmarkClock = { start: NaN, stop: NaN };
-  const b: BenchmarkTimer = createBenchmarkTimer(clock);
+  const b = createBenchmarkTimer(clock);
   // Iterating given benchmark definitions (await-in-loop)
   console.log(
     "running",
     benchmarks.length,
     `benchmark${benchmarks.length === 1 ? " ..." : "s ..."}`
   );
-  for (const { name, runs, func } of benchmarks) {
+  for (const { name, runs = 0, func } of benchmarks) {
     // See https://github.com/denoland/deno/pull/1452 about groupCollapsed
     console.groupCollapsed(`benchmark ${name} ... `);
     // Trying benchmark.func
-    let result: string;
+    let result = "";
     try {
       if (runs === 1) {
         // b is a benchmark timer interfacing an unset (NaN) benchmark clock
@@ -126,7 +126,7 @@ export async function runBenchmarks({
       } else if (runs > 1) {
         // Averaging runs
         let pendingRuns = runs;
-        let totalMs: number = 0;
+        let totalMs = 0;
         // Would be better 2 not run these serially
         while (true) {
           // b is a benchmark timer interfacing an unset (NaN) benchmark clock
diff --git a/benching/test.ts b/benching/test.ts
index 81cbc5e6f6..34f7b00d29 100644
--- a/benching/test.ts
+++ b/benching/test.ts
@@ -6,19 +6,19 @@ import "example.ts";
 test(async function benching() {
   bench(function forIncrementX1e9(b: BenchmarkTimer) {
     b.start();
-    for (let i: number = 0; i < 1e9; i++);
+    for (let i = 0; i < 1e9; i++);
     b.stop();
   });
 
   bench(function forDecrementX1e9(b: BenchmarkTimer) {
     b.start();
-    for (let i: number = 1e9; i > 0; i--);
+    for (let i = 1e9; i > 0; i--);
     b.stop();
   });
 
   bench(async function forAwaitFetchDenolandX10(b: BenchmarkTimer) {
     b.start();
-    for (let i: number = 0; i < 10; i++) {
+    for (let i = 0; i < 10; i++) {
       await fetch("https://deno.land/");
     }
     b.stop();
@@ -36,7 +36,7 @@ test(async function benching() {
     runs: 100,
     func(b: BenchmarkTimer) {
       b.start();
-      for (let i: number = 0; i < 1e6; i++);
+      for (let i = 0; i < 1e6; i++);
       b.stop();
     }
   });
diff --git a/bytes/bytes.ts b/bytes/bytes.ts
index ef333288e7..080ec4445d 100644
--- a/bytes/bytes.ts
+++ b/bytes/bytes.ts
@@ -22,7 +22,7 @@ export function bytesFindIndex(a: Uint8Array, pat: Uint8Array): number {
 }
 
 /** Find last index of binary pattern from a. If not found, then return -1 **/
-export function bytesFindLastIndex(a: Uint8Array, pat: Uint8Array) {
+export function bytesFindLastIndex(a: Uint8Array, pat: Uint8Array): number {
   const e = pat[pat.length - 1];
   for (let i = a.length - 1; i >= 0; i--) {
     if (a[i] !== e) continue;
diff --git a/colors/mod.ts b/colors/mod.ts
index 7a45905ce0..d0b4bc3200 100644
--- a/colors/mod.ts
+++ b/colors/mod.ts
@@ -9,7 +9,7 @@ interface Code {
 
 let enabled = !noColor;
 
-export function setEnabled(value: boolean) {
+export function setEnabled(value: boolean): void {
   if (noColor) {
     return;
   }
@@ -29,7 +29,7 @@ function code(open: number, close: number): Code {
   };
 }
 
-function run(str: string, code: Code) {
+function run(str: string, code: Code): string {
   return enabled
     ? `${code.open}${str.replace(code.regexp, code.open)}${code.close}`
     : str;
diff --git a/datetime/mod.ts b/datetime/mod.ts
index f30fc89c99..96ed3a860a 100644
--- a/datetime/mod.ts
+++ b/datetime/mod.ts
@@ -13,13 +13,13 @@ export function parseDate(dateStr: string, format: DateFormat): Date {
 
   if (format === "mm-dd-yyyy") {
     const datePattern = /^(\d{2})-(\d{2})-(\d{4})$/;
-    [, m, d, y] = datePattern.exec(dateStr);
+    [, m, d, y] = datePattern.exec(dateStr)!;
   } else if (format === "dd-mm-yyyy") {
     const datePattern = /^(\d{2})-(\d{2})-(\d{4})$/;
-    [, d, m, y] = datePattern.exec(dateStr);
+    [, d, m, y] = datePattern.exec(dateStr)!;
   } else if (format === "yyyy-mm-dd") {
     const datePattern = /^(\d{4})-(\d{2})-(\d{2})$/;
-    [, y, m, d] = datePattern.exec(dateStr);
+    [, y, m, d] = datePattern.exec(dateStr)!;
   } else {
     throw new Error("Invalid date format!");
   }
@@ -50,22 +50,22 @@ export function parseDateTime(
 
   if (format === "mm-dd-yyyy hh:mm") {
     const datePattern = /^(\d{2})-(\d{2})-(\d{4}) (\d{2}):(\d{2})$/;
-    [, m, d, y, ho, mi] = datePattern.exec(datetimeStr);
+    [, m, d, y, ho, mi] = datePattern.exec(datetimeStr)!;
   } else if (format === "dd-mm-yyyy hh:mm") {
     const datePattern = /^(\d{2})-(\d{2})-(\d{4}) (\d{2}):(\d{2})$/;
-    [, d, m, y, ho, mi] = datePattern.exec(datetimeStr);
+    [, d, m, y, ho, mi] = datePattern.exec(datetimeStr)!;
   } else if (format === "yyyy-mm-dd hh:mm") {
     const datePattern = /^(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2})$/;
-    [, y, m, d, ho, mi] = datePattern.exec(datetimeStr);
+    [, y, m, d, ho, mi] = datePattern.exec(datetimeStr)!;
   } else if (format === "hh:mm mm-dd-yyyy") {
     const datePattern = /^(\d{2}):(\d{2}) (\d{2})-(\d{2})-(\d{4})$/;
-    [, ho, mi, m, d, y] = datePattern.exec(datetimeStr);
+    [, ho, mi, m, d, y] = datePattern.exec(datetimeStr)!;
   } else if (format === "hh:mm dd-mm-yyyy") {
     const datePattern = /^(\d{2}):(\d{2}) (\d{2})-(\d{2})-(\d{4})$/;
-    [, ho, mi, d, m, y] = datePattern.exec(datetimeStr);
+    [, ho, mi, d, m, y] = datePattern.exec(datetimeStr)!;
   } else if (format === "hh:mm yyyy-mm-dd") {
     const datePattern = /^(\d{2}):(\d{2}) (\d{4})-(\d{2})-(\d{2})$/;
-    [, ho, mi, y, m, d] = datePattern.exec(datetimeStr);
+    [, ho, mi, y, m, d] = datePattern.exec(datetimeStr)!;
   } else {
     throw new Error("Invalid datetime format!");
   }
diff --git a/datetime/test.ts b/datetime/test.ts
index 3329e9cd1d..7182c27c41 100644
--- a/datetime/test.ts
+++ b/datetime/test.ts
@@ -31,6 +31,7 @@ test(function parseDateTime() {
 
 test(function invalidParseDateTimeFormatThrows() {
   try {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     (datetime as any).parseDateTime("2019-01-01 00:00", "x-y-z");
     assert(false, "no exception was thrown");
   } catch (e) {
@@ -55,6 +56,7 @@ test(function parseDate() {
 
 test(function invalidParseDateFormatThrows() {
   try {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     (datetime as any).parseDate("2019-01-01", "x-y-z");
     assert(false, "no exception was thrown");
   } catch (e) {
diff --git a/examples/gist.ts b/examples/gist.ts
index 8ce68a946e..f64f692262 100755
--- a/examples/gist.ts
+++ b/examples/gist.ts
@@ -9,7 +9,7 @@ function pathBase(p: string): string {
   return parts[parts.length - 1];
 }
 
-async function main() {
+async function main(): Promise<void> {
   const token = env()["GIST_TOKEN"];
   if (!token) {
     console.error("GIST_TOKEN environmental variable not set.");
@@ -46,7 +46,7 @@ async function main() {
     headers: [
       ["Content-Type", "application/json"],
       ["User-Agent", "Deno-Gist"],
-      ["Authorization", "token " + token]
+      ["Authorization", `token ${token}`]
     ],
     body
   });
diff --git a/examples/ws.ts b/examples/ws.ts
index bc2a7bd0be..f25a156875 100644
--- a/examples/ws.ts
+++ b/examples/ws.ts
@@ -6,7 +6,7 @@ import {
   isWebSocketPingEvent
 } from "https://deno.land/x/ws/mod.ts";
 
-async function main() {
+async function main(): Promise<void> {
   console.log("websocket server is running on 0.0.0.0:8080");
   for await (const req of serve("0.0.0.0:8080")) {
     if (req.url === "/ws") {
@@ -22,7 +22,7 @@ async function main() {
             // binary message
             console.log("ws:Binary", ev);
           } else if (isWebSocketPingEvent(ev)) {
-            const [_, body] = ev;
+            const [, body] = ev;
             // ping
             console.log("ws:Ping", body);
           } else if (isWebSocketCloseEvent(ev)) {
diff --git a/io/bufio.ts b/io/bufio.ts
index 05e6b0a0c7..33cddea2b0 100644
--- a/io/bufio.ts
+++ b/io/bufio.ts
@@ -211,7 +211,7 @@ export class BufReader implements Reader {
    * delim.
    * For simple uses, a Scanner may be more convenient.
    */
-  async readString(delim: string): Promise<string> {
+  async readString(_delim: string): Promise<string> {
     throw new Error("Not implemented");
   }
 
diff --git a/io/bufio_test.ts b/io/bufio_test.ts
index 0a261daea7..41151b6812 100644
--- a/io/bufio_test.ts
+++ b/io/bufio_test.ts
@@ -6,7 +6,7 @@
 const { Buffer } = Deno;
 import { Reader, ReadResult } from "deno";
 import { test, assert, assertEqual } from "../testing/mod.ts";
-import { BufReader, BufState, BufWriter } from "./bufio.ts";
+import { BufReader, BufWriter } from "./bufio.ts";
 import * as iotest from "./iotest.ts";
 import { charCode, copyBytes, stringsReader } from "./util.ts";
 
@@ -34,7 +34,10 @@ test(async function bufioReaderSimple() {
   assert.equal(s, data);
 });
 
-type ReadMaker = { name: string; fn: (r: Reader) => Reader };
+interface ReadMaker {
+  name: string;
+  fn: (r: Reader) => Reader;
+}
 
 const readMakers: ReadMaker[] = [
   { name: "full", fn: r => r },
@@ -44,18 +47,6 @@ const readMakers: ReadMaker[] = [
   // { name: "timeout", fn: r => new iotest.TimeoutReader(r) },
 ];
 
-function readLines(b: BufReader): string {
-  let s = "";
-  while (true) {
-    let s1 = b.readString("\n");
-    if (s1 == null) {
-      break; // EOF
-    }
-    s += s1;
-  }
-  return s;
-}
-
 // Call read to accumulate the text of a file
 async function reads(buf: BufReader, m: number): Promise<string> {
   const b = new Uint8Array(1000);
@@ -71,7 +62,10 @@ async function reads(buf: BufReader, m: number): Promise<string> {
   return decoder.decode(b.subarray(0, nb));
 }
 
-type NamedBufReader = { name: string; fn: (r: BufReader) => Promise<string> };
+interface NamedBufReader {
+  name: string;
+  fn: (r: BufReader) => Promise<string>;
+}
 
 const bufreaders: NamedBufReader[] = [
   { name: "1", fn: (b: BufReader) => reads(b, 1) },
@@ -187,6 +181,7 @@ async function testReadLine(input: Uint8Array): Promise<void> {
       if (err == "EOF") {
         break;
       }
+      // eslint-disable-next-line @typescript-eslint/restrict-plus-operands
       let want = testOutput.subarray(done, done + line.byteLength);
       assertEqual(
         line,
@@ -290,6 +285,7 @@ test(async function bufioWriter() {
   const data = new Uint8Array(8192);
 
   for (let i = 0; i < data.byteLength; i++) {
+    // eslint-disable-next-line @typescript-eslint/restrict-plus-operands
     data[i] = charCode(" ") + (i % (charCode("~") - charCode(" ")));
   }
 
diff --git a/io/ioutil_test.ts b/io/ioutil_test.ts
index 168aad52a0..08d0d677e8 100644
--- a/io/ioutil_test.ts
+++ b/io/ioutil_test.ts
@@ -1,7 +1,7 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 const { Buffer } = Deno;
 import { Reader, ReadResult } from "deno";
-import { assert, assertEqual, runTests, test } from "../testing/mod.ts";
+import { assert, assertEqual, test } from "../testing/mod.ts";
 import {
   copyN,
   readInt,
diff --git a/io/writers_test.ts b/io/writers_test.ts
index 94c58417df..ba4d7ed89b 100644
--- a/io/writers_test.ts
+++ b/io/writers_test.ts
@@ -7,7 +7,7 @@ import { copyN } from "./ioutil.ts";
 test(async function ioStringWriter() {
   const w = new StringWriter("base");
   const r = new StringReader("0123456789");
-  const n = await copyN(w, r, 4);
+  await copyN(w, r, 4);
   assert.equal(w.toString(), "base0123");
   await copy(w, r);
   assert.equal(w.toString(), "base0123456789");
diff --git a/log/handlers.ts b/log/handlers.ts
index 96d8c056fd..c1f664cc09 100644
--- a/log/handlers.ts
+++ b/log/handlers.ts
@@ -24,7 +24,7 @@ export class BaseHandler {
     this.formatter = options.formatter || DEFAULT_FORMATTER;
   }
 
-  handle(logRecord: LogRecord) {
+  handle(logRecord: LogRecord): void {
     if (this.level > logRecord.level) return;
 
     const msg = this.format(logRecord);
@@ -48,9 +48,9 @@ export class BaseHandler {
     });
   }
 
-  log(msg: string) {}
-  async setup() {}
-  async destroy() {}
+  log(_msg: string): void {}
+  async setup(): Promise<void> {}
+  async destroy(): Promise<void> {}
 }
 
 export class ConsoleHandler extends BaseHandler {
@@ -77,7 +77,7 @@ export class ConsoleHandler extends BaseHandler {
     return msg;
   }
 
-  log(msg: string) {
+  log(msg: string): void {
     console.log(msg);
   }
 }
@@ -86,7 +86,7 @@ export abstract class WriterHandler extends BaseHandler {
   protected _writer: Writer;
   private _encoder = new TextEncoder();
 
-  log(msg: string) {
+  log(msg: string): void {
     this._writer.write(this._encoder.encode(msg + "\n"));
   }
 }
@@ -104,13 +104,13 @@ export class FileHandler extends WriterHandler {
     this._filename = options.filename;
   }
 
-  async setup() {
+  async setup(): Promise<void> {
     // open file in append mode - write only
     this._file = await open(this._filename, "a");
     this._writer = this._file;
   }
 
-  async destroy() {
+  async destroy(): Promise<void> {
     await this._file.close();
   }
 }
diff --git a/log/handlers_test.ts b/log/handlers_test.ts
index 52eca16ea5..7dc4e2089d 100644
--- a/log/handlers_test.ts
+++ b/log/handlers_test.ts
@@ -1,6 +1,5 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import { assertEqual, test } from "../testing/mod.ts";
-import { LogRecord, Logger } from "./logger.ts";
 import { LogLevel, getLevelName, getLevelByName } from "./levels.ts";
 import { BaseHandler } from "./handlers.ts";
 
diff --git a/log/logger.ts b/log/logger.ts
index 678d5ed94d..d92c34a582 100644
--- a/log/logger.ts
+++ b/log/logger.ts
@@ -4,6 +4,7 @@ import { BaseHandler } from "./handlers.ts";
 
 export interface LogRecord {
   msg: string;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   args: any[];
   datetime: Date;
   level: number;
@@ -13,6 +14,7 @@ export interface LogRecord {
 export class Logger {
   level: number;
   levelName: string;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   handlers: any[];
 
   constructor(levelName: string, handlers?: BaseHandler[]) {
@@ -22,7 +24,8 @@ export class Logger {
     this.handlers = handlers || [];
   }
 
-  _log(level: number, msg: string, ...args: any[]) {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  _log(level: number, msg: string, ...args: any[]): void {
     if (this.level > level) return;
 
     // TODO: it'd be a good idea to make it immutable, so
@@ -41,23 +44,28 @@ export class Logger {
     });
   }
 
-  debug(msg: string, ...args: any[]) {
-    return this._log(LogLevel.DEBUG, msg, ...args);
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  debug(msg: string, ...args: any[]): void {
+    this._log(LogLevel.DEBUG, msg, ...args);
   }
 
-  info(msg: string, ...args: any[]) {
-    return this._log(LogLevel.INFO, msg, ...args);
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  info(msg: string, ...args: any[]): void {
+    this._log(LogLevel.INFO, msg, ...args);
   }
 
-  warning(msg: string, ...args: any[]) {
-    return this._log(LogLevel.WARNING, msg, ...args);
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  warning(msg: string, ...args: any[]): void {
+    this._log(LogLevel.WARNING, msg, ...args);
   }
 
-  error(msg: string, ...args: any[]) {
-    return this._log(LogLevel.ERROR, msg, ...args);
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  error(msg: string, ...args: any[]): void {
+    this._log(LogLevel.ERROR, msg, ...args);
   }
 
-  critical(msg: string, ...args: any[]) {
-    return this._log(LogLevel.CRITICAL, msg, ...args);
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  critical(msg: string, ...args: any[]): void {
+    this._log(LogLevel.CRITICAL, msg, ...args);
   }
 }
diff --git a/log/logger_test.ts b/log/logger_test.ts
index a2275a00bd..8ff328c484 100644
--- a/log/logger_test.ts
+++ b/log/logger_test.ts
@@ -53,7 +53,7 @@ test(function customHandler() {
 test(function logFunctions() {
   let handler: TestHandler;
 
-  const doLog = (level: string) => {
+  const doLog = (level: string): void => {
     handler = new TestHandler(level);
     let logger = new Logger(level, [handler]);
     logger.debug("foo");
diff --git a/log/mod.ts b/log/mod.ts
index 96dc81ff1a..ef5ca5a89b 100644
--- a/log/mod.ts
+++ b/log/mod.ts
@@ -36,8 +36,8 @@ const DEFAULT_CONFIG: LogConfig = {
 };
 
 const state = {
-  handlers: new Map(),
-  loggers: new Map(),
+  handlers: new Map<string, BaseHandler>(),
+  loggers: new Map<string, Logger>(),
   config: DEFAULT_CONFIG
 };
 
@@ -48,18 +48,7 @@ export const handlers = {
   FileHandler
 };
 
-export const debug = (msg: string, ...args: any[]) =>
-  getLogger("default").debug(msg, ...args);
-export const info = (msg: string, ...args: any[]) =>
-  getLogger("default").info(msg, ...args);
-export const warning = (msg: string, ...args: any[]) =>
-  getLogger("default").warning(msg, ...args);
-export const error = (msg: string, ...args: any[]) =>
-  getLogger("default").error(msg, ...args);
-export const critical = (msg: string, ...args: any[]) =>
-  getLogger("default").critical(msg, ...args);
-
-export function getLogger(name?: string) {
+export function getLogger(name?: string): Logger {
   if (!name) {
     return state.loggers.get("default");
   }
@@ -73,11 +62,27 @@ export function getLogger(name?: string) {
   return state.loggers.get(name);
 }
 
-export function getHandler(name: string) {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const debug = (msg: string, ...args: any[]): void =>
+  getLogger("default").debug(msg, ...args);
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const info = (msg: string, ...args: any[]): void =>
+  getLogger("default").info(msg, ...args);
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const warning = (msg: string, ...args: any[]): void =>
+  getLogger("default").warning(msg, ...args);
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const error = (msg: string, ...args: any[]): void =>
+  getLogger("default").error(msg, ...args);
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const critical = (msg: string, ...args: any[]): void =>
+  getLogger("default").critical(msg, ...args);
+
+export function getHandler(name: string): BaseHandler {
   return state.handlers.get(name);
 }
 
-export async function setup(config: LogConfig) {
+export async function setup(config: LogConfig): Promise<void> {
   state.config = {
     handlers: { ...DEFAULT_CONFIG.handlers, ...config.handlers },
     loggers: { ...DEFAULT_CONFIG.loggers, ...config.loggers }
@@ -106,7 +111,7 @@ export async function setup(config: LogConfig) {
   for (const loggerName in loggers) {
     const loggerConfig = loggers[loggerName];
     const handlerNames = loggerConfig.handlers || [];
-    const handlers = [];
+    const handlers: BaseHandler[] = [];
 
     handlerNames.forEach(handlerName => {
       if (state.handlers.has(handlerName)) {
diff --git a/log/test.ts b/log/test.ts
index 918368b97f..3599247bd2 100644
--- a/log/test.ts
+++ b/log/test.ts
@@ -1,8 +1,6 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import { assertEqual, test } from "../testing/mod.ts";
 import * as log from "./mod.ts";
-import { BaseHandler } from "./handlers.ts";
-import { LogRecord } from "./logger.ts";
 import { LogLevel } from "./levels.ts";
 
 import "./handlers_test.ts";
@@ -18,7 +16,7 @@ import "./logger_test.ts";
 class TestHandler extends log.handlers.BaseHandler {
   public messages: string[] = [];
 
-  log(msg: string) {
+  log(msg: string): void {
     this.messages.push(msg);
   }
 }
diff --git a/media_types/mod.ts b/media_types/mod.ts
index 3b74427ba3..b7c75fb223 100644
--- a/media_types/mod.ts
+++ b/media_types/mod.ts
@@ -42,7 +42,7 @@ export const types = new Map<string, string>();
 function populateMaps(
   extensions: Map<string, string[]>,
   types: Map<string, string>
-) {
+): void {
   const preference = ["nginx", "apache", undefined, "iana"];
 
   for (const type of Object.keys(db)) {
@@ -98,6 +98,17 @@ export function charset(type: string): string | undefined {
   }
 }
 
+/** Given an extension, lookup the appropriate media type for that extension.
+ * Likely you should be using `contentType()` though instead.
+ */
+export function lookup(path: string): string | undefined {
+  const extension = extname("x." + path)
+    .toLowerCase()
+    .substr(1);
+
+  return types.get(extension);
+}
+
 /** Given an extension or media type, return the full `Content-Type` header
  * string.  Returns `undefined` if not resolvable.
  */
@@ -136,14 +147,3 @@ export function extension(type: string): string | undefined {
 
   return exts[0];
 }
-
-/** Given an extension, lookup the appropriate media type for that extension.
- * Likely you should be using `contentType()` though instead.
- */
-export function lookup(path: string): string | undefined {
-  const extension = extname("x." + path)
-    .toLowerCase()
-    .substr(1);
-
-  return types.get(extension);
-}
diff --git a/multipart/formfile.ts b/multipart/formfile.ts
index b1b63eb157..8bbdcffbca 100644
--- a/multipart/formfile.ts
+++ b/multipart/formfile.ts
@@ -1,7 +1,7 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 
 /** FormFile object */
-export type FormFile = {
+export interface FormFile {
   /** filename  */
   filename: string;
   /** content-type header value of file */
@@ -12,10 +12,11 @@ export type FormFile = {
   content?: Uint8Array;
   /** temporal file path. Set if file size is bigger than specified max-memory size at reading form */
   tempfile?: string;
-};
+}
 
 /** Type guard for FormFile */
-export function isFormFile(x): x is FormFile {
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export function isFormFile(x: any): x is FormFile {
   return (
     typeof x === "object" &&
     x.hasOwnProperty("filename") &&
diff --git a/multipart/multipart.ts b/multipart/multipart.ts
index 70874ed85b..68785c7c6c 100644
--- a/multipart/multipart.ts
+++ b/multipart/multipart.ts
@@ -17,7 +17,7 @@ import { TextProtoReader } from "../textproto/mod.ts";
 import { encoder } from "../strings/strings.ts";
 import * as path from "../fs/path.ts";
 
-function randomBoundary() {
+function randomBoundary(): string {
   let boundary = "--------------------------";
   for (let i = 0; i < 24; i++) {
     boundary += Math.floor(Math.random() * 10).toString(16);
@@ -25,6 +25,185 @@ function randomBoundary() {
   return boundary;
 }
 
+export function matchAfterPrefix(
+  a: Uint8Array,
+  prefix: Uint8Array,
+  bufState: BufState
+): number {
+  if (a.length === prefix.length) {
+    if (bufState) {
+      return 1;
+    }
+    return 0;
+  }
+  const c = a[prefix.length];
+  if (
+    c === " ".charCodeAt(0) ||
+    c === "\t".charCodeAt(0) ||
+    c === "\r".charCodeAt(0) ||
+    c === "\n".charCodeAt(0) ||
+    c === "-".charCodeAt(0)
+  ) {
+    return 1;
+  }
+  return -1;
+}
+
+export function scanUntilBoundary(
+  buf: Uint8Array,
+  dashBoundary: Uint8Array,
+  newLineDashBoundary: Uint8Array,
+  total: number,
+  state: BufState
+): [number, BufState] {
+  if (total === 0) {
+    if (bytesHasPrefix(buf, dashBoundary)) {
+      switch (matchAfterPrefix(buf, dashBoundary, state)) {
+        case -1:
+          return [dashBoundary.length, null];
+        case 0:
+          return [0, null];
+        case 1:
+          return [0, "EOF"];
+      }
+      if (bytesHasPrefix(dashBoundary, buf)) {
+        return [0, state];
+      }
+    }
+  }
+  const i = bytesFindIndex(buf, newLineDashBoundary);
+  if (i >= 0) {
+    switch (matchAfterPrefix(buf.slice(i), newLineDashBoundary, state)) {
+      case -1:
+        // eslint-disable-next-line @typescript-eslint/restrict-plus-operands
+        return [i + newLineDashBoundary.length, null];
+      case 0:
+        return [i, null];
+      case 1:
+        return [i, "EOF"];
+    }
+  }
+  if (bytesHasPrefix(newLineDashBoundary, buf)) {
+    return [0, state];
+  }
+  const j = bytesFindLastIndex(buf, newLineDashBoundary.slice(0, 1));
+  if (j >= 0 && bytesHasPrefix(newLineDashBoundary, buf.slice(j))) {
+    return [j, null];
+  }
+  return [buf.length, state];
+}
+
+let i = 0;
+
+class PartReader implements Reader, Closer {
+  n: number = 0;
+  total: number = 0;
+  bufState: BufState = null;
+  index = i++;
+
+  constructor(private mr: MultipartReader, public readonly headers: Headers) {}
+
+  async read(p: Uint8Array): Promise<ReadResult> {
+    const br = this.mr.bufReader;
+    const returnResult = (nread: number, bufState: BufState): ReadResult => {
+      if (bufState && bufState !== "EOF") {
+        throw bufState;
+      }
+      return { nread, eof: bufState === "EOF" };
+    };
+    if (this.n === 0 && !this.bufState) {
+      const [peek] = await br.peek(br.buffered());
+      const [n, state] = scanUntilBoundary(
+        peek,
+        this.mr.dashBoundary,
+        this.mr.newLineDashBoundary,
+        this.total,
+        this.bufState
+      );
+      this.n = n;
+      this.bufState = state;
+      if (this.n === 0 && !this.bufState) {
+        // eslint-disable-next-line @typescript-eslint/restrict-plus-operands
+        const [, state] = await br.peek(peek.length + 1);
+        this.bufState = state;
+        if (this.bufState === "EOF") {
+          this.bufState = new RangeError("unexpected eof");
+        }
+      }
+    }
+    if (this.n === 0) {
+      return returnResult(0, this.bufState);
+    }
+
+    let n = 0;
+    if (p.byteLength > this.n) {
+      n = this.n;
+    }
+    const buf = p.slice(0, n);
+    const [nread] = await this.mr.bufReader.readFull(buf);
+    p.set(buf);
+    this.total += nread;
+    this.n -= nread;
+    if (this.n === 0) {
+      return returnResult(n, this.bufState);
+    }
+    return returnResult(n, null);
+  }
+
+  close(): void {}
+
+  private contentDisposition: string;
+  private contentDispositionParams: { [key: string]: string };
+
+  private getContentDispositionParams(): { [key: string]: string } {
+    if (this.contentDispositionParams) return this.contentDispositionParams;
+    const cd = this.headers.get("content-disposition");
+    const params = {};
+    const comps = cd.split(";");
+    this.contentDisposition = comps[0];
+    comps
+      .slice(1)
+      .map(v => v.trim())
+      .map(kv => {
+        const [k, v] = kv.split("=");
+        if (v) {
+          const s = v.charAt(0);
+          const e = v.charAt(v.length - 1);
+          if ((s === e && s === '"') || s === "'") {
+            params[k] = v.substr(1, v.length - 2);
+          } else {
+            params[k] = v;
+          }
+        }
+      });
+    return (this.contentDispositionParams = params);
+  }
+
+  get fileName(): string {
+    return this.getContentDispositionParams()["filename"];
+  }
+
+  get formName(): string {
+    const p = this.getContentDispositionParams();
+    if (this.contentDisposition === "form-data") {
+      return p["name"];
+    }
+    return "";
+  }
+}
+
+function skipLWSPChar(u: Uint8Array): Uint8Array {
+  const ret = new Uint8Array(u.length);
+  const sp = " ".charCodeAt(0);
+  const ht = "\t".charCodeAt(0);
+  let j = 0;
+  for (let i = 0; i < u.length; i++) {
+    if (u[i] === sp || u[i] === ht) continue;
+    ret[j++] = u[i];
+  }
+  return ret.slice(0, j);
+}
+
 /** Reader for parsing multipart/form-data */
 export class MultipartReader {
   readonly newLine = encoder.encode("\r\n");
@@ -125,7 +304,7 @@ export class MultipartReader {
         break;
       }
       if (state) {
-        throw new Error("aa" + state.toString());
+        throw new Error(`aa${state.toString()}`);
       }
       if (this.isBoundaryDelimiterLine(line)) {
         this.partsRead++;
@@ -155,7 +334,7 @@ export class MultipartReader {
     }
   }
 
-  private isFinalBoundary(line: Uint8Array) {
+  private isFinalBoundary(line: Uint8Array): boolean {
     if (!bytesHasPrefix(line, this.dashBoundaryDash)) {
       return false;
     }
@@ -163,7 +342,7 @@ export class MultipartReader {
     return rest.length === 0 || bytesEqual(skipLWSPChar(rest), this.newLine);
   }
 
-  private isBoundaryDelimiterLine(line: Uint8Array) {
+  private isBoundaryDelimiterLine(line: Uint8Array): boolean {
     if (!bytesHasPrefix(line, this.dashBoundary)) {
       return false;
     }
@@ -172,183 +351,6 @@ export class MultipartReader {
   }
 }
 
-function skipLWSPChar(u: Uint8Array): Uint8Array {
-  const ret = new Uint8Array(u.length);
-  const sp = " ".charCodeAt(0);
-  const ht = "\t".charCodeAt(0);
-  let j = 0;
-  for (let i = 0; i < u.length; i++) {
-    if (u[i] === sp || u[i] === ht) continue;
-    ret[j++] = u[i];
-  }
-  return ret.slice(0, j);
-}
-
-let i = 0;
-
-class PartReader implements Reader, Closer {
-  n: number = 0;
-  total: number = 0;
-  bufState: BufState = null;
-  index = i++;
-
-  constructor(private mr: MultipartReader, public readonly headers: Headers) {}
-
-  async read(p: Uint8Array): Promise<ReadResult> {
-    const br = this.mr.bufReader;
-    const returnResult = (nread: number, bufState: BufState): ReadResult => {
-      if (bufState && bufState !== "EOF") {
-        throw bufState;
-      }
-      return { nread, eof: bufState === "EOF" };
-    };
-    if (this.n === 0 && !this.bufState) {
-      const [peek] = await br.peek(br.buffered());
-      const [n, state] = scanUntilBoundary(
-        peek,
-        this.mr.dashBoundary,
-        this.mr.newLineDashBoundary,
-        this.total,
-        this.bufState
-      );
-      this.n = n;
-      this.bufState = state;
-      if (this.n === 0 && !this.bufState) {
-        const [_, state] = await br.peek(peek.length + 1);
-        this.bufState = state;
-        if (this.bufState === "EOF") {
-          this.bufState = new RangeError("unexpected eof");
-        }
-      }
-    }
-    if (this.n === 0) {
-      return returnResult(0, this.bufState);
-    }
-
-    let n = 0;
-    if (p.byteLength > this.n) {
-      n = this.n;
-    }
-    const buf = p.slice(0, n);
-    const [nread] = await this.mr.bufReader.readFull(buf);
-    p.set(buf);
-    this.total += nread;
-    this.n -= nread;
-    if (this.n === 0) {
-      return returnResult(n, this.bufState);
-    }
-    return returnResult(n, null);
-  }
-
-  close(): void {}
-
-  private contentDisposition: string;
-  private contentDispositionParams: { [key: string]: string };
-
-  private getContentDispositionParams() {
-    if (this.contentDispositionParams) return this.contentDispositionParams;
-    const cd = this.headers.get("content-disposition");
-    const params = {};
-    const comps = cd.split(";");
-    this.contentDisposition = comps[0];
-    comps
-      .slice(1)
-      .map(v => v.trim())
-      .map(kv => {
-        const [k, v] = kv.split("=");
-        if (v) {
-          const s = v.charAt(0);
-          const e = v.charAt(v.length - 1);
-          if ((s === e && s === '"') || s === "'") {
-            params[k] = v.substr(1, v.length - 2);
-          } else {
-            params[k] = v;
-          }
-        }
-      });
-    return (this.contentDispositionParams = params);
-  }
-
-  get fileName(): string {
-    return this.getContentDispositionParams()["filename"];
-  }
-
-  get formName(): string {
-    const p = this.getContentDispositionParams();
-    if (this.contentDisposition === "form-data") {
-      return p["name"];
-    }
-    return "";
-  }
-}
-
-export function scanUntilBoundary(
-  buf: Uint8Array,
-  dashBoundary: Uint8Array,
-  newLineDashBoundary: Uint8Array,
-  total: number,
-  state: BufState
-): [number, BufState] {
-  if (total === 0) {
-    if (bytesHasPrefix(buf, dashBoundary)) {
-      switch (matchAfterPrefix(buf, dashBoundary, state)) {
-        case -1:
-          return [dashBoundary.length, null];
-        case 0:
-          return [0, null];
-        case 1:
-          return [0, "EOF"];
-      }
-      if (bytesHasPrefix(dashBoundary, buf)) {
-        return [0, state];
-      }
-    }
-  }
-  const i = bytesFindIndex(buf, newLineDashBoundary);
-  if (i >= 0) {
-    switch (matchAfterPrefix(buf.slice(i), newLineDashBoundary, state)) {
-      case -1:
-        return [i + newLineDashBoundary.length, null];
-      case 0:
-        return [i, null];
-      case 1:
-        return [i, "EOF"];
-    }
-  }
-  if (bytesHasPrefix(newLineDashBoundary, buf)) {
-    return [0, state];
-  }
-  const j = bytesFindLastIndex(buf, newLineDashBoundary.slice(0, 1));
-  if (j >= 0 && bytesHasPrefix(newLineDashBoundary, buf.slice(j))) {
-    return [j, null];
-  }
-  return [buf.length, state];
-}
-
-export function matchAfterPrefix(
-  a: Uint8Array,
-  prefix: Uint8Array,
-  bufState: BufState
-): number {
-  if (a.length === prefix.length) {
-    if (bufState) {
-      return 1;
-    }
-    return 0;
-  }
-  const c = a[prefix.length];
-  if (
-    c === " ".charCodeAt(0) ||
-    c === "\t".charCodeAt(0) ||
-    c === "\r".charCodeAt(0) ||
-    c === "\n".charCodeAt(0) ||
-    c === "-".charCodeAt(0)
-  ) {
-    return 1;
-  }
-  return -1;
-}
-
 class PartWriter implements Writer {
   closed = false;
   private readonly partHeader: string;
@@ -389,9 +391,9 @@ class PartWriter implements Writer {
   }
 }
 
-function checkBoundary(b: string) {
+function checkBoundary(b: string): string {
   if (b.length < 1 || b.length > 70) {
-    throw new Error("invalid boundary length: " + b.length);
+    throw new Error(`invalid boundary length: ${b.length}`);
   }
   const end = b.length - 1;
   for (let i = 0; i < end; i++) {
@@ -407,7 +409,7 @@ function checkBoundary(b: string) {
 export class MultipartWriter {
   private readonly _boundary: string;
 
-  get boundary() {
+  get boundary(): string {
     return this._boundary;
   }
 
@@ -462,12 +464,16 @@ export class MultipartWriter {
     return this.createPart(h);
   }
 
-  async writeField(field: string, value: string) {
+  async writeField(field: string, value: string): Promise<void> {
     const f = await this.createFormField(field);
     await f.write(encoder.encode(value));
   }
 
-  async writeFile(field: string, filename: string, file: Reader) {
+  async writeFile(
+    field: string,
+    filename: string,
+    file: Reader
+  ): Promise<void> {
     const f = await this.createFormFile(field, filename);
     await copy(f, file);
   }
@@ -477,7 +483,7 @@ export class MultipartWriter {
   }
 
   /** Close writer. No additional data can be writen to stream */
-  async close() {
+  async close(): Promise<void> {
     if (this.isClosed) {
       throw new Error("multipart: writer is closed");
     }
diff --git a/multipart/multipart_test.ts b/multipart/multipart_test.ts
index a87db60d70..b4c0f761d7 100644
--- a/multipart/multipart_test.ts
+++ b/multipart/multipart_test.ts
@@ -13,7 +13,6 @@ import { FormFile, isFormFile } from "./formfile.ts";
 import { StringWriter } from "../io/writers.ts";
 
 const e = new TextEncoder();
-const d = new TextDecoder();
 const boundary = "--abcde";
 const dashBoundary = e.encode("--" + boundary);
 const nlDashBoundary = e.encode("\r\n--" + boundary);
diff --git a/prettier/main.ts b/prettier/main.ts
index 70c2250868..286cba58c1 100755
--- a/prettier/main.ts
+++ b/prettier/main.ts
@@ -2,7 +2,7 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 // This script formats the given source files. If the files are omitted, it
 // formats the all files in the repository.
-const { args, platform, readAll, lstat, exit, run, readFile, writeFile } = Deno;
+const { args, readAll, lstat, exit, readFile, writeFile } = Deno;
 import { xrun } from "./util.ts";
 import { parse } from "../flags/mod.ts";
 import { prettier, prettierPlugins } from "./prettier.ts";
@@ -67,7 +67,7 @@ async function getSourceFiles(args: string[]): Promise<string[]> {
 }
 
 // Filters out the files which contains any pattern in the given ignoreList.
-function filterIgnoreList(files: string[], ignoreList: string[]) {
+function filterIgnoreList(files: string[], ignoreList: string[]): string[] {
   return files.filter(path =>
     ignoreList.every(pattern => !path.includes(pattern))
   );
@@ -203,7 +203,7 @@ async function formatSourceFiles(
   exit(0);
 }
 
-async function main(opts) {
+async function main(opts): Promise<void> {
   const { help, ignore, check, _: args } = opts;
 
   if (help) {
diff --git a/prettier/main_test.ts b/prettier/main_test.ts
index 165081e002..b6c0489eee 100644
--- a/prettier/main_test.ts
+++ b/prettier/main_test.ts
@@ -5,7 +5,9 @@ const { readAll } = Deno;
 
 const decoder = new TextDecoder();
 
-async function run(args: string[]) {
+async function run(
+  args: string[]
+): Promise<{ stdout: string; code: number | undefined }> {
   const p = xrun({ args, stdout: "piped" });
 
   const stdout = decoder.decode(await readAll(p.stdout));
@@ -33,7 +35,7 @@ function normalizeOutput(output: string): string {
     .join("\n");
 }
 
-async function clearTestdataChanges() {
+async function clearTestdataChanges(): Promise<void> {
   await xrun({ args: ["git", "checkout", testdata] }).status();
 }
 
diff --git a/prettier/prettier.ts b/prettier/prettier.ts
index 67099c5ca5..98f36a78dc 100644
--- a/prettier/prettier.ts
+++ b/prettier/prettier.ts
@@ -5,6 +5,7 @@ import "./vendor/parser_babylon.js";
 import "./vendor/parser_markdown.js";
 
 // TODO: provide decent type declarions for these
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
 const { prettier, prettierPlugins } = window as any;
 
 export { prettier, prettierPlugins };
diff --git a/prettier/util.ts b/prettier/util.ts
index 0be583f757..425cbfe966 100644
--- a/prettier/util.ts
+++ b/prettier/util.ts
@@ -2,7 +2,7 @@
 const { platform, run } = Deno;
 
 // Runs a command in cross-platform way
-export function xrun(opts) {
+export function xrun(opts): Deno.Process {
   return run({
     ...opts,
     args: platform.os === "win" ? ["cmd.exe", "/c", ...opts.args] : opts.args
diff --git a/testing/diff.ts b/testing/diff.ts
index a1385b88a0..4fab75e4a8 100644
--- a/testing/diff.ts
+++ b/testing/diff.ts
@@ -15,7 +15,7 @@ const REMOVED = 1;
 const COMMON = 2;
 const ADDED = 3;
 
-function createCommon<T>(A: T[], B: T[], reverse?: boolean) {
+function createCommon<T>(A: T[], B: T[], reverse?: boolean): T[] {
   const common = [];
   if (A.length === 0 || B.length === 0) return [];
   for (let i = 0; i < Math.min(A.length, B.length); i += 1) {
@@ -30,97 +30,7 @@ function createCommon<T>(A: T[], B: T[], reverse?: boolean) {
   return common;
 }
 
-export default function diff<T>(A: T[], B: T[]): DiffResult<T>[] {
-  function backTrace<T>(
-    A: T[],
-    B: T[],
-    current: FarthestPoint,
-    swapped: boolean
-  ) {
-    const M = A.length;
-    const N = B.length;
-    const result = [];
-    let a = M - 1;
-    let b = N - 1;
-    let j = routes[current.id];
-    let type = routes[current.id + diffTypesPtrOffset];
-    while (true) {
-      if (!j && !type) break;
-      const prev = j;
-      if (type === REMOVED) {
-        result.unshift({
-          type: (swapped ? "removed" : "added") as DiffType,
-          value: B[b]
-        });
-        b -= 1;
-      } else if (type === ADDED) {
-        result.unshift({
-          type: (swapped ? "added" : "removed") as DiffType,
-          value: A[a]
-        });
-        a -= 1;
-      } else {
-        result.unshift({ type: "common" as DiffType, value: A[a] });
-        a -= 1;
-        b -= 1;
-      }
-      j = routes[prev];
-      type = routes[prev + diffTypesPtrOffset];
-    }
-    return result;
-  }
-
-  function createFP(
-    slide: FarthestPoint,
-    down: FarthestPoint,
-    k: number,
-    M: number,
-    N: number
-  ): FarthestPoint {
-    if (slide && slide.y === -1 && (down && down.y === -1))
-      return { y: 0, id: 0 };
-    if (
-      (down && down.y === -1) ||
-      k === M ||
-      (slide && slide.y) > (down && down.y) + 1
-    ) {
-      const prev = slide.id;
-      ptr++;
-      routes[ptr] = prev;
-      routes[ptr + diffTypesPtrOffset] = ADDED;
-      return { y: slide.y, id: ptr };
-    } else {
-      const prev = down.id;
-      ptr++;
-      routes[ptr] = prev;
-      routes[ptr + diffTypesPtrOffset] = REMOVED;
-      return { y: down.y + 1, id: ptr };
-    }
-  }
-
-  function snake<T>(
-    k: number,
-    slide: FarthestPoint,
-    down: FarthestPoint,
-    offset: number,
-    A: T[],
-    B: T[]
-  ) {
-    const M = A.length;
-    const N = B.length;
-    if (k < -N || M < k) return { y: -1 };
-    const fp = createFP(slide, down, k, M, N);
-    while (fp.y + k < M && fp.y < N && A[fp.y + k] === B[fp.y]) {
-      const prev = fp.id;
-      ptr++;
-      fp.id = ptr;
-      fp.y += 1;
-      routes[ptr] = prev;
-      routes[ptr + diffTypesPtrOffset] = COMMON;
-    }
-    return fp;
-  }
-
+export default function diff<T>(A: T[], B: T[]): Array<DiffResult<T>> {
   const prefixCommon = createCommon(A, B);
   const suffixCommon = createCommon(
     A.slice(prefixCommon.length),
@@ -159,6 +69,99 @@ export default function diff<T>(A: T[], B: T[]): DiffResult<T>[] {
   const diffTypesPtrOffset = routes.length / 2;
   let ptr = 0;
   let p = -1;
+
+  function backTrace<T>(
+    A: T[],
+    B: T[],
+    current: FarthestPoint,
+    swapped: boolean
+  ): Array<{
+    type: DiffType;
+    value: T;
+  }> {
+    const M = A.length;
+    const N = B.length;
+    const result = [];
+    let a = M - 1;
+    let b = N - 1;
+    let j = routes[current.id];
+    let type = routes[current.id + diffTypesPtrOffset];
+    while (true) {
+      if (!j && !type) break;
+      const prev = j;
+      if (type === REMOVED) {
+        result.unshift({
+          type: (swapped ? "removed" : "added") as DiffType,
+          value: B[b]
+        });
+        b -= 1;
+      } else if (type === ADDED) {
+        result.unshift({
+          type: (swapped ? "added" : "removed") as DiffType,
+          value: A[a]
+        });
+        a -= 1;
+      } else {
+        result.unshift({ type: "common" as DiffType, value: A[a] });
+        a -= 1;
+        b -= 1;
+      }
+      j = routes[prev];
+      type = routes[prev + diffTypesPtrOffset];
+    }
+    return result;
+  }
+
+  function createFP(
+    slide: FarthestPoint,
+    down: FarthestPoint,
+    k: number,
+    M: number
+  ): FarthestPoint {
+    if (slide && slide.y === -1 && (down && down.y === -1))
+      return { y: 0, id: 0 };
+    if (
+      (down && down.y === -1) ||
+      k === M ||
+      (slide && slide.y) > (down && down.y) + 1
+    ) {
+      const prev = slide.id;
+      ptr++;
+      routes[ptr] = prev;
+      routes[ptr + diffTypesPtrOffset] = ADDED;
+      return { y: slide.y, id: ptr };
+    } else {
+      const prev = down.id;
+      ptr++;
+      routes[ptr] = prev;
+      routes[ptr + diffTypesPtrOffset] = REMOVED;
+      return { y: down.y + 1, id: ptr };
+    }
+  }
+
+  function snake<T>(
+    k: number,
+    slide: FarthestPoint,
+    down: FarthestPoint,
+    _offset: number,
+    A: T[],
+    B: T[]
+  ): FarthestPoint {
+    const M = A.length;
+    const N = B.length;
+    if (k < -N || M < k) return { y: -1, id: -1 };
+    const fp = createFP(slide, down, k, M);
+    while (fp.y + k < M && fp.y < N && A[fp.y + k] === B[fp.y]) {
+      const prev = fp.id;
+      ptr++;
+      fp.id = ptr;
+      fp.y += 1;
+      routes[ptr] = prev;
+      routes[ptr + diffTypesPtrOffset] = COMMON;
+    }
+    return fp;
+  }
+
   while (fp[delta + offset].y < N) {
     p = p + 1;
     for (let k = -p; k < delta; ++k) {
diff --git a/testing/format.ts b/testing/format.ts
index 8434db1c2d..9a8b02ac6a 100644
--- a/testing/format.ts
+++ b/testing/format.ts
@@ -6,6 +6,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  */
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
 export type Refs = any[];
 export type Optional<T> = { [K in keyof T]?: T[K] };
 
@@ -32,7 +33,7 @@ export interface Config {
 }
 
 export type Printer = (
-  val: any,
+  val: unknown,
   config: Config,
   indentation: string,
   depth: number,
@@ -66,12 +67,15 @@ interface BasicValueOptions {
  * Explicitly comparing typeof constructor to function avoids undefined as name
  * when mock identity-obj-proxy returns the key as the value for any key.
  */
-const getConstructorName = (val: new (...args: any[]) => any) =>
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const getConstructorName = (val: new (...args: any[]) => any): string =>
   (typeof val.constructor === "function" && val.constructor.name) || "Object";
 
 /* global window */
 /** Is val is equal to global window object? Works even if it does not exist :) */
-const isWindow = (val: any) => typeof window !== "undefined" && val === window;
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const isWindow = (val: any): val is Window =>
+  typeof window !== "undefined" && val === window;
 
 const SYMBOL_REGEXP = /^Symbol\((.*)\)(.*)$/;
 
@@ -116,11 +120,12 @@ function printError(val: Error): string {
  * data-types in JS.
  */
 function printBasicValue(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   val: any,
   { printFunctionName, escapeRegex, escapeString }: BasicValueOptions
 ): string | null {
   if (val === true || val === false) {
-    return "" + val;
+    return String(val);
   }
   if (val === undefined) {
     return "undefined";
@@ -136,9 +141,9 @@ function printBasicValue(
   }
   if (typeOf === "string") {
     if (escapeString) {
-      return '"' + val.replace(/"|\\/g, "\\$&") + '"';
+      return `"${val.replace(/"|\\/g, "\\$&")}"`;
     }
-    return '"' + val + '"';
+    return `"${val}"`;
   }
   if (typeOf === "function") {
     return printFunction(val, printFunctionName);
@@ -185,95 +190,8 @@ function printBasicValue(
   return null;
 }
 
-/**
- * Handles more complex objects ( such as objects with circular references.
- * maps and sets etc )
- */
-function printComplexValue(
-  val: any,
-  config: Config,
-  indentation: string,
-  depth: number,
-  refs: Refs,
-  hasCalledToJSON?: boolean
-): string {
-  if (refs.indexOf(val) !== -1) {
-    return "[Circular]";
-  }
-  refs = refs.slice();
-  refs.push(val);
-
-  const hitMaxDepth = ++depth > config.maxDepth;
-  const { min, callToJSON } = config;
-
-  if (
-    callToJSON &&
-    !hitMaxDepth &&
-    val.toJSON &&
-    typeof val.toJSON === "function" &&
-    !hasCalledToJSON
-  ) {
-    return printer(val.toJSON(), config, indentation, depth, refs, true);
-  }
-
-  const toStringed = toString.call(val);
-  if (toStringed === "[object Arguments]") {
-    return hitMaxDepth
-      ? "[Arguments]"
-      : (min ? "" : "Arguments ") +
-          "[" +
-          printListItems(val, config, indentation, depth, refs, printer) +
-          "]";
-  }
-  if (isToStringedArrayType(toStringed)) {
-    return hitMaxDepth
-      ? "[" + val.constructor.name + "]"
-      : (min ? "" : val.constructor.name + " ") +
-          "[" +
-          printListItems(val, config, indentation, depth, refs, printer) +
-          "]";
-  }
-  if (toStringed === "[object Map]") {
-    return hitMaxDepth
-      ? "[Map]"
-      : "Map {" +
-          printIteratorEntries(
-            val.entries(),
-            config,
-            indentation,
-            depth,
-            refs,
-            printer,
-            " => "
-          ) +
-          "}";
-  }
-  if (toStringed === "[object Set]") {
-    return hitMaxDepth
-      ? "[Set]"
-      : "Set {" +
-          printIteratorValues(
-            val.values(),
-            config,
-            indentation,
-            depth,
-            refs,
-            printer
-          ) +
-          "}";
-  }
-
-  // Avoid failure to serialize global window object in jsdom test environment.
-  // For example, not even relevant if window is prop of React element.
-  return hitMaxDepth || isWindow(val)
-    ? "[" + getConstructorName(val) + "]"
-    : (min ? "" : getConstructorName(val) + " ") +
-        "{" +
-        printObjectProperties(val, config, indentation, depth, refs, printer) +
-        "}";
-}
-
 function printer(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   val: any,
   config: Config,
   indentation: string,
@@ -285,6 +203,7 @@ function printer(
   if (basicResult !== null) {
     return basicResult;
   }
+  // eslint-disable-next-line @typescript-eslint/no-use-before-define
   return printComplexValue(
     val,
     config,
@@ -295,30 +214,44 @@ function printer(
   );
 }
 
-const getConfig = (options: Options): Config => ({
-  ...options,
-  indent: options.min ? "" : createIndent(options.indent),
-  spacingInner: options.min ? " " : "\n",
-  spacingOuter: options.min ? "" : "\n"
-});
+/**
+ * Return items (for example, of an array)
+ * with spacing, indentation, and comma
+ * without surrounding punctuation (for example, brackets)
+ */
+function printListItems(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  list: any,
+  config: Config,
+  indentation: string,
+  depth: number,
+  refs: Refs,
+  printer: Printer
+): string {
+  let result = "";
 
-function createIndent(indent: number): string {
-  return new Array(indent + 1).join(" ");
-}
+  if (list.length) {
+    result += config.spacingOuter;
 
-const getKeysOfEnumerableProperties = (object: {}) => {
-  const keys: Array<string | symbol> = Object.keys(object).sort();
+    const indentationNext = indentation + config.indent;
 
-  if (Object.getOwnPropertySymbols) {
-    Object.getOwnPropertySymbols(object).forEach(symbol => {
-      if (Object.getOwnPropertyDescriptor(object, symbol)!.enumerable) {
-        keys.push(symbol);
+    for (let i = 0; i < list.length; i++) {
+      result +=
+        indentationNext +
+        printer(list[i], config, indentationNext, depth, refs);
+
+      if (i < list.length - 1) {
+        result += "," + config.spacingInner;
+      } else if (!config.min) {
+        result += ",";
       }
-    });
+    }
+
+    result += config.spacingOuter + indentation;
   }
 
-  return keys;
-};
+  return result;
+}
 
 /**
  * Return entries (for example, of a map)
@@ -326,6 +259,7 @@ const getKeysOfEnumerableProperties = (object: {}) => {
  * without surrounding punctuation (for example, braces)
  */
 function printIteratorEntries(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   iterator: any,
   config: Config,
   indentation: string,
@@ -384,6 +318,7 @@ function printIteratorEntries(
  * without surrounding punctuation (braces or brackets)
  */
 function printIteratorValues(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   iterator: Iterator<any>,
   config: Config,
   indentation: string,
@@ -419,43 +354,19 @@ function printIteratorValues(
   return result;
 }
 
-/**
- * Return items (for example, of an array)
- * with spacing, indentation, and comma
- * without surrounding punctuation (for example, brackets)
- */
-function printListItems(
-  list: any,
-  config: Config,
-  indentation: string,
-  depth: number,
-  refs: Refs,
-  printer: Printer
-): string {
-  let result = "";
+const getKeysOfEnumerableProperties = (object: {}): Array<string | symbol> => {
+  const keys: Array<string | symbol> = Object.keys(object).sort();
 
-  if (list.length) {
-    result += config.spacingOuter;
-
-    const indentationNext = indentation + config.indent;
-
-    for (let i = 0; i < list.length; i++) {
-      result +=
-        indentationNext +
-        printer(list[i], config, indentationNext, depth, refs);
-
-      if (i < list.length - 1) {
-        result += "," + config.spacingInner;
-      } else if (!config.min) {
-        result += ",";
+  if (Object.getOwnPropertySymbols) {
+    Object.getOwnPropertySymbols(object).forEach(symbol => {
+      if (Object.getOwnPropertyDescriptor(object, symbol)!.enumerable) {
+        keys.push(symbol);
       }
-    }
-
-    result += config.spacingOuter + indentation;
+    });
   }
 
-  return result;
-}
+  return keys;
+};
 
 /**
  * Return properties of an object
@@ -504,11 +415,113 @@ function printObjectProperties(
   return result;
 }
 
+/**
+ * Handles more complex objects ( such as objects with circular references.
+ * maps and sets etc )
+ */
+function printComplexValue(
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  val: any,
+  config: Config,
+  indentation: string,
+  depth: number,
+  refs: Refs,
+  hasCalledToJSON?: boolean
+): string {
+  if (refs.indexOf(val) !== -1) {
+    return "[Circular]";
+  }
+  refs = refs.slice();
+  refs.push(val);
+
+  const hitMaxDepth = ++depth > config.maxDepth;
+  const { min, callToJSON } = config;
+
+  if (
+    callToJSON &&
+    !hitMaxDepth &&
+    val.toJSON &&
+    typeof val.toJSON === "function" &&
+    !hasCalledToJSON
+  ) {
+    return printer(val.toJSON(), config, indentation, depth, refs, true);
+  }
+
+  const toStringed = toString.call(val);
+  if (toStringed === "[object Arguments]") {
+    return hitMaxDepth
+      ? "[Arguments]"
+      : (min ? "" : "Arguments ") +
+          "[" +
+          printListItems(val, config, indentation, depth, refs, printer) +
+          "]";
+  }
+  if (isToStringedArrayType(toStringed)) {
+    return hitMaxDepth
+      ? `[${val.constructor.name}]`
+      : (min ? "" : `${val.constructor.name} `) +
+          "[" +
+          printListItems(val, config, indentation, depth, refs, printer) +
+          "]";
+  }
+  if (toStringed === "[object Map]") {
+    return hitMaxDepth
+      ? "[Map]"
+      : "Map {" +
+          printIteratorEntries(
+            val.entries(),
+            config,
+            indentation,
+            depth,
+            refs,
+            printer,
+            " => "
+          ) +
+          "}";
+  }
+  if (toStringed === "[object Set]") {
+    return hitMaxDepth
+      ? "[Set]"
+      : "Set {" +
+          printIteratorValues(
+            val.values(),
+            config,
+            indentation,
+            depth,
+            refs,
+            printer
+          ) +
+          "}";
+  }
+
+  // Avoid failure to serialize global window object in jsdom test environment.
+  // For example, not even relevant if window is prop of React element.
+  return hitMaxDepth || isWindow(val)
+    ? "[" + getConstructorName(val) + "]"
+    : (min ? "" : getConstructorName(val) + " ") +
+        "{" +
+        printObjectProperties(val, config, indentation, depth, refs, printer) +
+        "}";
+}
+
+// TODO this is better done with `.padStart()`
+function createIndent(indent: number): string {
+  return new Array(indent + 1).join(" ");
+}
+
+const getConfig = (options: Options): Config => ({
+  ...options,
+  indent: options.min ? "" : createIndent(options.indent),
+  spacingInner: options.min ? " " : "\n",
+  spacingOuter: options.min ? "" : "\n"
+});
+
 /**
  * Returns a presentation string of your `val` object
  * @param val any potential JavaScript object
  * @param options Custom settings
  */
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
 export function format(val: any, options: Optional<Options> = {}): string {
   const opts = Object.keys(DEFAULT_OPTIONS).reduce(
     (acc: Options, k: keyof Options) => {
diff --git a/testing/format_test.ts b/testing/format_test.ts
index 3e6da44802..7ca0235a50 100644
--- a/testing/format_test.ts
+++ b/testing/format_test.ts
@@ -9,17 +9,19 @@
 import { test, assertEqual } from "./mod.ts";
 import { format } from "./format.ts";
 
-function returnArguments(..._args: Array<unknown>) {
+// eslint-disable-next-line @typescript-eslint/no-unused-vars,@typescript-eslint/no-explicit-any
+function returnArguments(...args: any[]): IArguments {
   return arguments;
 }
 
-function MyObject(value: unknown) {
+function MyObject(value: unknown): void {
   // @ts-ignore
   this.name = value;
 }
 
 class MyArray<T> extends Array<T> {}
 
+// eslint-disable-next-line @typescript-eslint/explicit-function-return-type
 const createVal = () => [
   {
     id: "8658c1d0-9eda-4a90-95e1-8001e8eb6036",
@@ -32,6 +34,7 @@ const createVal = () => [
   }
 ];
 
+// eslint-disable-next-line @typescript-eslint/explicit-function-return-type
 const createExpected = () =>
   [
     "Array [",
@@ -58,7 +61,7 @@ test({
 test({
   name: "prints an empty array",
   fn() {
-    const val: any[] = [];
+    const val: unknown[] = [];
     assertEqual(format(val), "Array []");
   }
 });
@@ -151,7 +154,7 @@ test({
   name: "prints an anonymous callback function",
   fn() {
     let val;
-    function f(cb: () => void) {
+    function f(cb: () => void): void {
       val = cb;
     }
     // tslint:disable-next-line:no-empty
@@ -164,7 +167,7 @@ test({
   name: "prints an anonymous assigned function",
   fn() {
     // tslint:disable-next-line:no-empty
-    const val = () => {};
+    const val = (): void => {};
     const formatted = format(val);
     assertEqual(
       formatted === "[Function anonymous]" || formatted === "[Function val]",
@@ -177,7 +180,7 @@ test({
   name: "prints a named function",
   fn() {
     // tslint:disable-next-line:no-empty
-    const val = function named() {};
+    const val = function named(): void {};
     assertEqual(format(val), "[Function named]");
   }
 });
@@ -185,7 +188,7 @@ test({
 test({
   name: "prints a named generator function",
   fn() {
-    const val = function* generate() {
+    const val = function* generate(): IterableIterator<number> {
       yield 1;
       yield 2;
       yield 3;
@@ -198,7 +201,7 @@ test({
   name: "can customize function names",
   fn() {
     // tslint:disable-next-line:no-empty
-    const val = function named() {};
+    const val = function named(): void {};
     assertEqual(
       format(val, {
         printFunctionName: false
@@ -248,6 +251,7 @@ test({
 test({
   name: "prints a map with non-string keys",
   fn() {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const val = new Map<any, any>([
       [false, "boolean"],
       ["false", "string"],
@@ -373,6 +377,7 @@ test({
 test({
   name: "prints an object with properties and symbols",
   fn() {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const val: any = {};
     val[Symbol("symbol1")] = "value2";
     val[Symbol("symbol2")] = "value3";
@@ -388,7 +393,7 @@ test({
   name:
     "prints an object without non-enumerable properties which have string key",
   fn() {
-    const val: any = {
+    const val = {
       enumerable: true
     };
     const key = "non-enumerable";
@@ -404,7 +409,7 @@ test({
   name:
     "prints an object without non-enumerable properties which have symbol key",
   fn() {
-    const val: any = {
+    const val = {
       enumerable: true
     };
     const key = Symbol("non-enumerable");
@@ -609,6 +614,7 @@ test({
 test({
   name: "prints circular references",
   fn() {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const val: any = {};
     val.prop = val;
     assertEqual(format(val), 'Object {\n  "prop": [Circular],\n}');
@@ -787,6 +793,7 @@ test({
   name: "calls toJSON on Sets",
   fn() {
     const set = new Set([1]);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
     (set as any).toJSON = () => "map";
     assertEqual(format(set), '"map"');
   }
diff --git a/testing/mod.ts b/testing/mod.ts
index 41ae61bea3..c1bf2015b1 100644
--- a/testing/mod.ts
+++ b/testing/mod.ts
@@ -3,9 +3,37 @@
 import { green, red } from "../colors/mod.ts";
 
 interface Constructor {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
   new (...args: any[]): any;
 }
 
+export function equal(c: unknown, d: unknown): boolean {
+  const seen = new Map();
+  return (function compare(a: unknown, b: unknown) {
+    if (Object.is(a, b)) {
+      return true;
+    }
+    if (a && typeof a === "object" && b && typeof b === "object") {
+      if (seen.get(a) === b) {
+        return true;
+      }
+      if (Object.keys(a || {}).length !== Object.keys(b || {}).length) {
+        return false;
+      }
+      const merged = { ...a, ...b };
+      for (const key in merged) {
+        type Key = keyof typeof merged;
+        if (!compare(a && a[key as Key], b && b[key as Key])) {
+          return false;
+        }
+      }
+      seen.set(a, b);
+      return true;
+    }
+    return false;
+  })(c, d);
+}
+
 const assertions = {
   /** Make an assertion, if not `true`, then throw. */
   assert(expr: boolean, msg = ""): void {
@@ -78,6 +106,7 @@ const assertions = {
    * Forcefully throws a failed assertion
    */
   fail(msg?: string): void {
+    // eslint-disable-next-line @typescript-eslint/no-use-before-define
     assert(false, `Failed assertion${msg ? `: ${msg}` : "."}`);
   },
 
@@ -163,33 +192,6 @@ export const assert = assertions.assert as Assert;
  */
 export const assertEqual = assert.equal;
 
-export function equal(c: unknown, d: unknown): boolean {
-  const seen = new Map();
-  return (function compare(a: unknown, b: unknown) {
-    if (Object.is(a, b)) {
-      return true;
-    }
-    if (a && typeof a === "object" && b && typeof b === "object") {
-      if (seen.get(a) === b) {
-        return true;
-      }
-      if (Object.keys(a || {}).length !== Object.keys(b || {}).length) {
-        return false;
-      }
-      const merged = { ...a, ...b };
-      for (const key in merged) {
-        type Key = keyof typeof merged;
-        if (!compare(a && a[key as Key], b && b[key as Key])) {
-          return false;
-        }
-      }
-      seen.set(a, b);
-      return true;
-    }
-    return false;
-  })(c, d);
-}
-
 export type TestFunction = () => void | Promise<void>;
 
 export interface TestDefinition {
@@ -203,14 +205,20 @@ let filterRegExp: RegExp | null;
 const tests: TestDefinition[] = [];
 
 let filtered = 0;
-const ignored = 0;
-const measured = 0;
 
 // Must be called before any test() that needs to be filtered.
 export function setFilter(s: string): void {
   filterRegExp = new RegExp(s, "i");
 }
 
+function filter(name: string): boolean {
+  if (filterRegExp) {
+    return filterRegExp.test(name);
+  } else {
+    return true;
+  }
+}
+
 export function test(t: TestDefinition | TestFunction): void {
   const fn: TestFunction = typeof t === "function" ? t : t.fn;
   const name: string = t.name;
@@ -225,21 +233,8 @@ export function test(t: TestDefinition | TestFunction): void {
   }
 }
 
-function filter(name: string): boolean {
-  if (filterRegExp) {
-    return filterRegExp.test(name);
-  } else {
-    return true;
-  }
-}
-
-function red_failed() {
-  return red("FAILED");
-}
-
-function green_ok() {
-  return green("ok");
-}
+const RED_FAILED = red("FAILED");
+const GREEN_OK = green("ok");
 
 interface TestStats {
   filtered: number;
@@ -261,7 +256,7 @@ interface TestResults {
   cases: Map<number, TestResult>;
 }
 
-function createTestResults(tests: Array<TestDefinition>): TestResults {
+function createTestResults(tests: TestDefinition[]): TestResults {
   return tests.reduce(
     (acc: TestResults, { name }: TestDefinition, i: number): TestResults => {
       acc.keys.set(name, i);
@@ -274,10 +269,10 @@ function createTestResults(tests: Array<TestDefinition>): TestResults {
 
 function report(result: TestResult): void {
   if (result.ok) {
-    console.log(`test ${result.name} ... ${green_ok()}`);
+    console.log(`test ${result.name} ... ${GREEN_OK}`);
   } else if (result.error) {
     console.error(
-      `test ${result.name} ... ${red_failed()}\n${result.error.stack}`
+      `test ${result.name} ... ${RED_FAILED}\n${result.error.stack}`
     );
   } else {
     console.log(`test ${result.name} ... unresolved`);
@@ -302,7 +297,7 @@ function printResults(
   }
   // Attempting to match the output of Rust's test runner.
   console.log(
-    `\ntest result: ${stats.failed ? red_failed() : green_ok()}. ` +
+    `\ntest result: ${stats.failed ? RED_FAILED : GREEN_OK}. ` +
       `${stats.passed} passed; ${stats.failed} failed; ` +
       `${stats.ignored} ignored; ${stats.measured} measured; ` +
       `${stats.filtered} filtered out\n`
@@ -342,7 +337,7 @@ async function createTestCase(
 function initTestCases(
   stats: TestStats,
   results: TestResults,
-  tests: Array<TestDefinition>
+  tests: TestDefinition[]
 ): Array<Promise<void>> {
   return tests.map(createTestCase.bind(null, stats, results));
 }
@@ -350,7 +345,7 @@ function initTestCases(
 async function runTestsParallel(
   stats: TestStats,
   results: TestResults,
-  tests: Array<TestDefinition>
+  tests: TestDefinition[]
 ): Promise<void> {
   try {
     await Promise.all(initTestCases(stats, results, tests));
@@ -362,7 +357,7 @@ async function runTestsParallel(
 
 async function runTestsSerial(
   stats: TestStats,
-  tests: Array<TestDefinition>
+  tests: TestDefinition[]
 ): Promise<void> {
   for (const { fn, name } of tests) {
     // See https://github.com/denoland/deno/pull/1452
@@ -371,10 +366,10 @@ async function runTestsSerial(
     try {
       await fn();
       stats.passed++;
-      console.log("...", green_ok());
+      console.log("...", GREEN_OK);
       console.groupEnd();
     } catch (err) {
-      console.log("...", red_failed());
+      console.log("...", RED_FAILED);
       console.groupEnd();
       console.error(err.stack);
       stats.failed++;
diff --git a/testing/pretty.ts b/testing/pretty.ts
index aa90f24693..b27b3ccd75 100644
--- a/testing/pretty.ts
+++ b/testing/pretty.ts
@@ -15,7 +15,7 @@ function createStr(v: unknown): string {
   }
 }
 
-function createColor(diffType: DiffType) {
+function createColor(diffType: DiffType): (s: string) => string {
   switch (diffType) {
     case "added":
       return (s: string) => green(bold(s));
@@ -26,7 +26,7 @@ function createColor(diffType: DiffType) {
   }
 }
 
-function createSign(diffType: DiffType) {
+function createSign(diffType: DiffType): string {
   switch (diffType) {
     case "added":
       return "+   ";
@@ -37,7 +37,7 @@ function createSign(diffType: DiffType) {
   }
 }
 
-function buildMessage(diffResult: ReadonlyArray<DiffResult<string>>) {
+function buildMessage(diffResult: ReadonlyArray<DiffResult<string>>): string[] {
   const messages = [];
   messages.push("");
   messages.push("");
@@ -55,7 +55,7 @@ function buildMessage(diffResult: ReadonlyArray<DiffResult<string>>) {
   return messages;
 }
 
-export function assertEqual(actual: unknown, expected: unknown) {
+export function assertEqual(actual: unknown, expected: unknown): void {
   if (equal(actual, expected)) {
     return;
   }
diff --git a/testing/pretty_test.ts b/testing/pretty_test.ts
index f3b087aff9..89b35c0882 100644
--- a/testing/pretty_test.ts
+++ b/testing/pretty_test.ts
@@ -4,7 +4,7 @@ import { test, assert } from "./mod.ts";
 import { red, green, white, gray, bold } from "../colors/mod.ts";
 import { assertEqual } from "./pretty.ts";
 
-const createHeader = () => [
+const createHeader = (): string[] => [
   "",
   "",
   `    ${gray(bold("[Diff]"))} ${red(bold("Left"))} / ${green(bold("Right"))}`,
@@ -12,8 +12,8 @@ const createHeader = () => [
   ""
 ];
 
-const added = (s: string) => green(bold(s));
-const removed = (s: string) => red(bold(s));
+const added: (s: string) => string = (s: string): string => green(bold(s));
+const removed: (s: string) => string = (s: string): string => red(bold(s));
 
 test({
   name: "pass case",
diff --git a/testing/test.ts b/testing/test.ts
index 0d79c22184..7182a5783e 100644
--- a/testing/test.ts
+++ b/testing/test.ts
@@ -35,8 +35,6 @@ test(function testingAssertEqual() {
 });
 
 test(function testingAssertFail() {
-  let didThrow = false;
-
   assert.throws(assert.fail, Error, "Failed assertion.");
   assert.throws(
     () => {
diff --git a/textproto/mod.ts b/textproto/mod.ts
index ee76472967..28eac4609a 100644
--- a/textproto/mod.ts
+++ b/textproto/mod.ts
@@ -22,6 +22,17 @@ export class ProtocolError extends Error {
   }
 }
 
+export function append(a: Uint8Array, b: Uint8Array): Uint8Array {
+  if (a == null) {
+    return b;
+  } else {
+    const output = new Uint8Array(a.length + b.length);
+    output.set(a, 0);
+    output.set(b, a.length);
+    return output;
+  }
+}
+
 export class TextProtoReader {
   constructor(readonly r: BufReader) {}
 
@@ -137,14 +148,3 @@ export class TextProtoReader {
     return [line, null];
   }
 }
-
-export function append(a: Uint8Array, b: Uint8Array): Uint8Array {
-  if (a == null) {
-    return b;
-  } else {
-    const output = new Uint8Array(a.length + b.length);
-    output.set(a, 0);
-    output.set(b, a.length);
-    return output;
-  }
-}
diff --git a/textproto/test.ts b/textproto/test.ts
index 0f8bee2275..b6a4c93c9b 100644
--- a/textproto/test.ts
+++ b/textproto/test.ts
@@ -92,7 +92,7 @@ test(async function textprotoAppend() {
 
 test(async function textprotoReadEmpty() {
   let r = reader("");
-  let [m, err] = await r.readMIMEHeader();
+  let [, err] = await r.readMIMEHeader();
   // Should not crash!
   assertEqual(err, "EOF");
 });
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000000..5a8dca33d0
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "allowJs": true,
+    "baseUrl": ".",
+    "module": "esnext",
+    "moduleResolution": "node",
+    "noEmit": true,
+    "noLib": true,
+    "pretty": true,
+    "resolveJsonModule": true,
+    "strict": true,
+    "target": "esnext"
+  },
+  "include": ["./**/*.ts"]
+}