diff --git a/archive/tar.ts b/archive/tar.ts
index 5ef3aa2a7596..c72abc65988c 100644
--- a/archive/tar.ts
+++ b/archive/tar.ts
@@ -35,8 +35,10 @@ const ustar = "ustar\u000000";
  * Simple file reader
  */
 export class FileReader implements Deno.Reader {
-  private file: Deno.File;
+  private file?: Deno.File;
+
   constructor(private filePath: string, private mode: Deno.OpenMode = "r") {}
+
   public async read(p: Uint8Array): Promise<Deno.ReadResult> {
     if (!this.file) {
       this.file = await Deno.open(this.filePath, this.mode);
@@ -44,7 +46,7 @@ export class FileReader implements Deno.Reader {
     const res = await Deno.read(this.file.rid, p);
     if (res.eof) {
       await Deno.close(this.file.rid);
-      this.file = null;
+      this.file = undefined;
     }
     return res;
   }
@@ -54,18 +56,21 @@ export class FileReader implements Deno.Reader {
  * Simple file writer (call FileWriter.dispose() after use)
  */
 export class FileWriter implements Deno.Writer {
-  private file: Deno.File;
+  private file?: Deno.File;
+
   constructor(private filePath: string, private mode: Deno.OpenMode = "w") {}
+
   public async write(p: Uint8Array): Promise<number> {
     if (!this.file) {
       this.file = await Deno.open(this.filePath, this.mode);
     }
     return Deno.write(this.file.rid, p);
   }
+
   public dispose(): void {
     if (!this.file) return;
     Deno.close(this.file.rid);
-    this.file = null;
+    this.file = undefined;
   }
 }
 
@@ -191,7 +196,7 @@ function formatHeader(data: TarData): Uint8Array {
     buffer = clean(512);
   let offset = 0;
   ustarStructure.forEach(function(value): void {
-    const entry = encoder.encode(data[value.field] || "");
+    const entry = encoder.encode(data[value.field as keyof TarData] || "");
     buffer.set(entry, offset);
     offset += value.length; // space it out with nulls
   });
@@ -307,12 +312,13 @@ export class Tar {
         }
         i--;
       }
-      if (i < 0 || fileName.length > 100 || fileNamePrefix.length > 155) {
+      if (i < 0 || fileName.length > 100 || fileNamePrefix!.length > 155) {
         throw new Error(
           "ustar format does not allow a long file name (length of [file name prefix] + / + [file name] must be shorter than 256 bytes)"
         );
       }
     }
+    fileNamePrefix = fileNamePrefix!;
 
     opts = opts || {};
 
@@ -344,7 +350,7 @@ export class Tar {
       fileMode: pad(mode, 7),
       uid: pad(uid, 7),
       gid: pad(gid, 7),
-      fileSize: pad(info ? info.len : opts.contentSize, 11),
+      fileSize: pad((info ? info.len : opts.contentSize)!, 11),
       mtime: pad(mtime, 11),
       checksum: "        ",
       type: "0", // just a file
@@ -362,7 +368,7 @@ export class Tar {
       .filter((key): boolean => ["filePath", "reader"].indexOf(key) < 0)
       .forEach(function(key): void {
         checksum += encoder
-          .encode(tarData[key])
+          .encode(tarData[key as keyof TarData])
           .reduce((p, c): number => p + c, 0);
       });
 
@@ -381,7 +387,7 @@ export class Tar {
           headerArr = formatHeader(tarData);
         readers.push(new Deno.Buffer(headerArr));
         if (!reader) {
-          reader = new FileReader(filePath);
+          reader = new FileReader(filePath!);
         }
         readers.push(reader);
 
@@ -390,7 +396,7 @@ export class Tar {
           new Deno.Buffer(
             clean(
               recordSize -
-                (parseInt(tarData.fileSize, 8) % recordSize || recordSize)
+                (parseInt(tarData.fileSize!, 8) % recordSize || recordSize)
             )
           )
         );
@@ -451,7 +457,7 @@ export class Untar {
       (key): void => {
         const arr = trim(header[key]);
         if (arr.byteLength > 0) {
-          meta[key] = parseInt(decoder.decode(arr), 8);
+          meta[key as keyof UntarOptions] = parseInt(decoder.decode(arr), 8);
         }
       }
     );
@@ -459,7 +465,7 @@ export class Untar {
       (key): void => {
         const arr = trim(header[key]);
         if (arr.byteLength > 0) {
-          meta[key] = decoder.decode(arr);
+          meta[key as keyof UntarOptions] = decoder.decode(arr);
         }
       }
     );
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index ccad44e4f74c..b993f7aafb0c 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -14,7 +14,7 @@ jobs:
       - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
       - script: npx eslint **/*.ts --max-warnings=0
       - script: deno run --allow-run --allow-write --allow-read format.ts --check
-      - script: deno run --allow-run --allow-net --allow-write --allow-read test.ts
+      - script: deno run --allow-run --allow-net --allow-write --allow-read --config=tsconfig.test.json test.ts
 
   - job: "Mac"
     pool:
@@ -26,7 +26,7 @@ jobs:
       - script: echo '##vso[task.prependpath]$(HOME)/.deno/bin/'
       - script: eslint **/*.ts --max-warnings=0
       - script: deno run --allow-run --allow-write --allow-read format.ts --check
-      - script: deno run --allow-run --allow-net --allow-write --allow-read test.ts
+      - script: deno run --allow-run --allow-net --allow-write --allow-read --config=tsconfig.test.json test.ts
 
   - job: "Windows"
     pool:
@@ -37,4 +37,4 @@ jobs:
       - bash: echo "##vso[task.prependpath]C:\Users\VssAdministrator\.deno\\bin"
       - bash: npx eslint **/*.ts --max-warnings=0
       - bash: deno.exe run --allow-run --allow-write --allow-read format.ts --check
-      - bash: deno.exe run --allow-run --allow-net --allow-write --allow-read test.ts
+      - bash: deno.exe run --allow-run --allow-net --allow-write --allow-read --config=tsconfig.test.json test.ts
diff --git a/encoding/csv.ts b/encoding/csv.ts
index 191961ace284..aa2ceb1cf053 100644
--- a/encoding/csv.ts
+++ b/encoding/csv.ts
@@ -28,7 +28,7 @@ export interface ParseOptions {
 function chkOptions(opt: ParseOptions): void {
   if (
     INVALID_RUNE.includes(opt.comma) ||
-    INVALID_RUNE.includes(opt.comment) ||
+    (opt.comment && INVALID_RUNE.includes(opt.comment)) ||
     opt.comma === opt.comment
   ) {
     throw new Error("Invalid Delimiter");
@@ -130,7 +130,7 @@ export async function readAll(
     }
 
     if (lineResult.length > 0) {
-      if (_nbFields && _nbFields !== lineResult.length) {
+      if (_nbFields! && _nbFields! !== lineResult.length) {
         throw new ParseError(lineIndex, lineIndex, "wrong number of fields");
       }
       result.push(lineResult);
diff --git a/encoding/csv_test.ts b/encoding/csv_test.ts
index 40a2abcef9a9..0cf95b473a7b 100644
--- a/encoding/csv_test.ts
+++ b/encoding/csv_test.ts
@@ -441,7 +441,7 @@ for (const t of testCases) {
       if (t.Error) {
         let err;
         try {
-          actual = await readAll(new BufReader(new StringReader(t.Input)), {
+          actual = await readAll(new BufReader(new StringReader(t.Input!)), {
             comma: comma,
             comment: comment,
             trimLeadingSpace: trim,
@@ -454,7 +454,7 @@ for (const t of testCases) {
         assert(err);
         assertEquals(err.message, t.Error);
       } else {
-        actual = await readAll(new BufReader(new StringReader(t.Input)), {
+        actual = await readAll(new BufReader(new StringReader(t.Input!)), {
           comma: comma,
           comment: comment,
           trimLeadingSpace: trim,
diff --git a/encoding/toml.ts b/encoding/toml.ts
index cc96322fb2fc..b47e31bba77a 100644
--- a/encoding/toml.ts
+++ b/encoding/toml.ts
@@ -3,20 +3,19 @@ import { deepAssign } from "../util/deep_assign.ts";
 import { pad } from "../strings/pad.ts";
 
 class KeyValuePair {
-  key: string;
-  value: unknown;
+  constructor(public key: string, public value: unknown) {}
 }
 
 class ParserGroup {
-  type: string;
-  name: string;
   arrValues: unknown[] = [];
-  objValues: object = {};
+  objValues: Record<string, unknown> = {};
+
+  constructor(public type: string, public name: string) {}
 }
 
 class ParserContext {
   currentGroup?: ParserGroup;
-  output: object = {};
+  output: Record<string, unknown> = {};
 }
 
 class Parser {
@@ -27,7 +26,7 @@ class Parser {
     this.context = new ParserContext();
   }
   _sanitize(): void {
-    const out = [];
+    const out: string[] = [];
     for (let i = 0; i < this.tomlLines.length; i++) {
       const s = this.tomlLines[i];
       const trimmed = s.trim();
@@ -124,28 +123,30 @@ class Parser {
     this.tomlLines = merged;
   }
   _unflat(keys: string[], values: object = {}, cObj: object = {}): object {
-    let out = {};
+    let out: Record<string, unknown> = {};
     if (keys.length === 0) {
       return cObj;
     } else {
       if (Object.keys(cObj).length === 0) {
         cObj = values;
       }
-      let key = keys.pop();
-      out[key] = cObj;
+      let key: string | undefined = keys.pop();
+      if (key) {
+        out[key] = cObj;
+      }
       return this._unflat(keys, values, out);
     }
   }
   _groupToOutput(): void {
-    const arrProperty = this.context.currentGroup.name
-      .replace(/"/g, "")
+    const arrProperty = this.context
+      .currentGroup!.name.replace(/"/g, "")
       .replace(/'/g, "")
       .split(".");
     let u = {};
-    if (this.context.currentGroup.type === "array") {
-      u = this._unflat(arrProperty, this.context.currentGroup.arrValues);
+    if (this.context.currentGroup!.type === "array") {
+      u = this._unflat(arrProperty, this.context.currentGroup!.arrValues);
     } else {
-      u = this._unflat(arrProperty, this.context.currentGroup.objValues);
+      u = this._unflat(arrProperty, this.context.currentGroup!.objValues);
     }
     deepAssign(this.context.output, u);
     delete this.context.currentGroup;
@@ -167,22 +168,22 @@ class Parser {
     if (this.context.currentGroup) {
       this._groupToOutput();
     }
-    let g = new ParserGroup();
-    g.name = line.match(captureReg)[1];
-    if (g.name.match(/\[.*\]/)) {
-      g.type = "array";
-      g.name = g.name.match(captureReg)[1];
+
+    let type;
+    let name = line.match(captureReg)![1];
+    if (name.match(/\[.*\]/)) {
+      type = "array";
+      name = name.match(captureReg)![1];
     } else {
-      g.type = "object";
+      type = "object";
     }
-    this.context.currentGroup = g;
+    this.context.currentGroup = new ParserGroup(type, name);
   }
   _processDeclaration(line: string): KeyValuePair {
-    let kv = new KeyValuePair();
     const idx = line.indexOf("=");
-    kv.key = line.substring(0, idx).trim();
-    kv.value = this._parseData(line.slice(idx + 1));
-    return kv;
+    const key = line.substring(0, idx).trim();
+    const value = this._parseData(line.slice(idx + 1));
+    return new KeyValuePair(key, value);
   }
   // TODO (zekth) Need refactor using ACC
   _parseData(dataString: string): unknown {
@@ -353,23 +354,27 @@ class Parser {
   _cleanOutput(): void {
     this._propertyClean(this.context.output);
   }
-  _propertyClean(obj: object): void {
+  _propertyClean(obj: Record<string, unknown>): void {
     const keys = Object.keys(obj);
     for (let i = 0; i < keys.length; i++) {
       let k = keys[i];
-      let v = obj[k];
-      let pathDeclaration = this._parseDeclarationName(k);
-      delete obj[k];
-      if (pathDeclaration.length > 1) {
-        k = pathDeclaration.shift();
-        k = k.replace(/"/g, "");
-        v = this._unflat(pathDeclaration, v as object);
-      } else {
-        k = k.replace(/"/g, "");
-      }
-      obj[k] = v;
-      if (v instanceof Object) {
-        this._propertyClean(v);
+      if (k) {
+        let v = obj[k];
+        let pathDeclaration = this._parseDeclarationName(k);
+        delete obj[k];
+        if (pathDeclaration.length > 1) {
+          const shift = pathDeclaration.shift();
+          if (shift) {
+            k = shift.replace(/"/g, "");
+            v = this._unflat(pathDeclaration, v as object);
+          }
+        } else {
+          k = k.replace(/"/g, "");
+        }
+        obj[k] = v;
+        if (v instanceof Object) {
+          this._propertyClean(v);
+        }
       }
     }
   }
@@ -393,18 +398,26 @@ class Dumper {
     this.output = this._format();
     return this.output;
   }
-  _parse(obj: object, path: string = ""): string[] {
+  _parse(obj: Record<string, unknown>, path: string = ""): string[] {
     const out = [];
     const props = Object.keys(obj);
     const propObj = props.filter(
-      (e): boolean =>
-        (obj[e] instanceof Array && !this._isSimplySerializable(obj[e][0])) ||
-        !this._isSimplySerializable(obj[e])
+      (e: string): boolean => {
+        if (obj[e] instanceof Array) {
+          const d: unknown[] = obj[e] as unknown[];
+          return !this._isSimplySerializable(d[0]);
+        }
+        return !this._isSimplySerializable(obj[e]);
+      }
     );
     const propPrim = props.filter(
-      (e): boolean =>
-        !(obj[e] instanceof Array && !this._isSimplySerializable(obj[e][0])) &&
-        this._isSimplySerializable(obj[e])
+      (e: string): boolean => {
+        if (obj[e] instanceof Array) {
+          const d: unknown[] = obj[e] as unknown[];
+          return this._isSimplySerializable(d[0]);
+        }
+        return this._isSimplySerializable(obj[e]);
+      }
     );
     const k = propPrim.concat(propObj);
     for (let i = 0; i < k.length; i++) {
@@ -435,7 +448,11 @@ class Dumper {
       } else if (typeof value === "object") {
         out.push("");
         out.push(this._header(path + prop));
-        out.push(...this._parse(value, `${path}${prop}.`));
+        if (value) {
+          const toParse: Record<string, unknown> = value;
+          out.push(...this._parse(toParse, `${path}${prop}.`));
+        }
+        // out.push(...this._parse(value, `${path}${prop}.`));
       }
     }
     out.push("");
diff --git a/flags/all_bool_test.ts b/flags/all_bool_test.ts
index 22b6bcd2d23d..711bf3da1adb 100755
--- a/flags/all_bool_test.ts
+++ b/flags/all_bool_test.ts
@@ -19,7 +19,7 @@ test(function flagBooleanTrue(): void {
 
 // flag boolean true only affects double hyphen arguments without equals signs
 test(function flagBooleanTrueOnlyAffectsDoubleDash(): void {
-  var argv = parse(["moo", "--honk", "cow", "-p", "55", "--tacos=good"], {
+  const argv = parse(["moo", "--honk", "cow", "-p", "55", "--tacos=good"], {
     boolean: true
   });
 
diff --git a/flags/dotted_test.ts b/flags/dotted_test.ts
index 6a27eb6f7707..e5c69ce6df54 100755
--- a/flags/dotted_test.ts
+++ b/flags/dotted_test.ts
@@ -8,17 +8,18 @@ test(function dottedAlias(): void {
     default: { "a.b": 11 },
     alias: { "a.b": "aa.bb" }
   });
+  console.log(argv.a);
   assertEquals(argv.a.b, 22);
   assertEquals(argv.aa.bb, 22);
 });
 
 test(function dottedDefault(): void {
-  const argv = parse("", { default: { "a.b": 11 }, alias: { "a.b": "aa.bb" } });
+  const argv = parse([], { default: { "a.b": 11 }, alias: { "a.b": "aa.bb" } });
   assertEquals(argv.a.b, 11);
   assertEquals(argv.aa.bb, 11);
 });
 
 test(function dottedDefaultWithNoAlias(): void {
-  const argv = parse("", { default: { "a.b": 11 } });
+  const argv = parse([], { default: { "a.b": 11 } });
   assertEquals(argv.a.b, 11);
 });
diff --git a/flags/mod.ts b/flags/mod.ts
index 433f4eb154df..7b4451a02d4c 100644
--- a/flags/mod.ts
+++ b/flags/mod.ts
@@ -1,16 +1,16 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 export interface ArgParsingOptions {
-  unknown?: Function;
+  unknown?: (i: unknown) => unknown;
   boolean?: boolean | string | string[];
   alias?: { [key: string]: string | string[] };
   string?: string | string[];
-  default?: { [key: string]: any }; // eslint-disable-line @typescript-eslint/no-explicit-any
+  default?: { [key: string]: unknown }; // eslint-disable-line @typescript-eslint/no-explicit-any
   "--"?: boolean;
   stopEarly?: boolean;
 }
 
 const DEFAULT_OPTIONS = {
-  unknown: (i): unknown => i,
+  unknown: (i: unknown): unknown => i,
   boolean: false,
   alias: {},
   string: [],
@@ -19,16 +19,27 @@ const DEFAULT_OPTIONS = {
   stopEarly: false
 };
 
+interface Flags {
+  bools: { [key: string]: boolean };
+  strings: { [key: string]: boolean };
+  unknownFn: (i: unknown) => unknown;
+  allBools: boolean;
+}
+
+interface NestedMapping {
+  [key: string]: NestedMapping | unknown;
+}
+
 function isNumber(x: unknown): boolean {
   if (typeof x === "number") return true;
   if (/^0x[0-9a-f]+$/i.test(String(x))) return true;
   return /^[-+]?(?:\d+(?:\.\d*)?|\.\d+)(e[-+]?\d+)?$/.test(String(x));
 }
 
-function hasKey(obj, keys): boolean {
+function hasKey(obj: NestedMapping, keys: string[]): boolean {
   let o = obj;
-  keys.slice(0, -1).forEach(function(key): void {
-    o = o[key] || {};
+  keys.slice(0, -1).forEach(function(key: string): void {
+    o = (o[key] || {}) as NestedMapping;
   });
 
   const key = keys[keys.length - 1];
@@ -36,7 +47,8 @@ function hasKey(obj, keys): boolean {
 }
 
 export function parse(
-  args,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  args: any[],
   initialOptions?: ArgParsingOptions
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
 ): { [key: string]: any } {
@@ -45,65 +57,85 @@ export function parse(
     ...(initialOptions || {})
   };
 
-  const flags = {
+  const flags: Flags = {
     bools: {},
     strings: {},
     unknownFn: options.unknown!,
     allBools: false
   };
 
-  // TODO: get rid of this, providing two different options
-  if (typeof options["boolean"] === "boolean" && options["boolean"]) {
-    flags.allBools = true;
-  } else {
-    []
-      .concat(options["boolean"])
-      .filter(Boolean)
-      .forEach(function(key): void {
-        flags.bools[key] = true;
-      });
+  if (options.boolean !== undefined) {
+    if (typeof options.boolean === "boolean") {
+      flags.allBools = !!options.boolean;
+    } else {
+      const booleanArgs: string[] =
+        typeof options.boolean === "string"
+          ? [options.boolean]
+          : options.boolean;
+
+      booleanArgs.filter(Boolean).forEach(
+        (key: string): void => {
+          flags.bools[key] = true;
+        }
+      );
+    }
   }
 
-  const aliases = {};
-  Object.keys(options.alias).forEach(function(key): void {
-    aliases[key] = [].concat(options.alias[key]);
-    aliases[key].forEach(function(x): void {
-      aliases[x] = [key].concat(
-        aliases[key].filter(function(y): boolean {
-          return x !== y;
-        })
-      );
-    });
-  });
+  const aliases: { [key: string]: string[] } = {};
+  if (options.alias !== undefined) {
+    for (const key in options.alias) {
+      const val = options.alias[key];
+
+      if (typeof val === "string") {
+        aliases[key] = [val];
+      } else {
+        aliases[key] = val;
+      }
 
-  []
-    .concat(options.string)
-    .filter(Boolean)
-    .forEach(function(key): void {
+      for (const alias of aliases[key]) {
+        aliases[alias] = [key].concat(
+          aliases[key].filter((y: string): boolean => alias !== y)
+        );
+      }
+    }
+  }
+
+  if (options.string !== undefined) {
+    const stringArgs =
+      typeof options.string === "string" ? [options.string] : options.string;
+
+    stringArgs.filter(Boolean).forEach(function(key): void {
       flags.strings[key] = true;
       if (aliases[key]) {
-        flags.strings[aliases[key]] = true;
+        aliases[key].forEach(
+          (alias: string): void => {
+            flags.strings[alias] = true;
+          }
+        );
       }
     });
+  }
 
   const defaults = options.default!;
 
-  const argv = { _: [] };
+  const argv: { [key: string]: unknown[] } = { _: [] };
 
-  function argDefined(key, arg): boolean {
+  function argDefined(key: string, arg: string): boolean {
     return (
       (flags.allBools && /^--[^=]+$/.test(arg)) ||
-      flags.strings[key] ||
       flags.bools[key] ||
-      aliases[key]
+      !!flags.strings[key] ||
+      !!aliases[key]
     );
   }
 
-  function setKey(obj, keys, value): void {
+  function setKey(obj: NestedMapping, keys: string[], value: unknown): void {
     let o = obj;
     keys.slice(0, -1).forEach(function(key): void {
-      if (o[key] === undefined) o[key] = {};
-      o = o[key];
+      if (o[key] === undefined) {
+        o[key] = {};
+      }
+      o = o[key] as NestedMapping;
     });
 
     const key = keys[keys.length - 1];
@@ -114,13 +146,17 @@ export function parse(
     ) {
       o[key] = value;
     } else if (Array.isArray(o[key])) {
-      o[key].push(value);
+      (o[key] as unknown[]).push(value);
     } else {
       o[key] = [o[key], value];
     }
   }
 
-  function setArg(key, val, arg = null): void {
+  function setArg(
+    key: string,
+    val: unknown,
+    arg: string | undefined = undefined
+  ): void {
     if (arg && flags.unknownFn && !argDefined(key, arg)) {
       if (flags.unknownFn(arg) === false) return;
     }
@@ -133,7 +169,7 @@ export function parse(
     });
   }
 
-  function aliasIsBoolean(key): boolean {
+  function aliasIsBoolean(key: string): boolean {
     return aliases[key].some(function(x): boolean {
       return flags.bools[x];
     });
@@ -143,8 +179,9 @@ export function parse(
     setArg(key, defaults[key] === undefined ? false : defaults[key]);
   });
 
-  let notFlags = [];
+  let notFlags: string[] = [];
 
+  // all args after "--" are not parsed
   if (args.indexOf("--") !== -1) {
     notFlags = args.slice(args.indexOf("--") + 1);
     args = args.slice(0, args.indexOf("--"));
@@ -157,18 +194,21 @@ export function parse(
       // Using [\s\S] instead of . because js doesn't support the
       // 'dotall' regex modifier. See:
       // http://stackoverflow.com/a/1068308/13216
-      const m = arg.match(/^--([^=]+)=([\s\S]*)$/);
+      const m = arg.match(/^--([^=]+)=([\s\S]*)$/)!;
       const key = m[1];
-      let value = m[2];
+      const value = m[2];
+
       if (flags.bools[key]) {
-        value = value !== "false";
+        const booleanValue = value !== "false";
+        setArg(key, booleanValue, arg);
+      } else {
+        setArg(key, value, arg);
       }
-      setArg(key, value, arg);
     } else if (/^--no-.+/.test(arg)) {
-      const key = arg.match(/^--no-(.+)/)[1];
+      const key = arg.match(/^--no-(.+)/)![1];
       setArg(key, false, arg);
     } else if (/^--.+/.test(arg)) {
-      const key = arg.match(/^--(.+)/)[1];
+      const key = arg.match(/^--(.+)/)![1];
       const next = args[i + 1];
       if (
         next !== undefined &&
diff --git a/flags/unknown_test.ts b/flags/unknown_test.ts
index 84d4db8995bb..dde725a75d4a 100755
--- a/flags/unknown_test.ts
+++ b/flags/unknown_test.ts
@@ -4,8 +4,8 @@ import { assertEquals } from "../testing/asserts.ts";
 import { parse } from "./mod.ts";
 
 test(function booleanAndAliasIsNotUnknown(): void {
-  const unknown = [];
-  function unknownFn(arg): boolean {
+  const unknown: unknown[] = [];
+  function unknownFn(arg: unknown): boolean {
     unknown.push(arg);
     return false;
   }
@@ -23,8 +23,8 @@ test(function booleanAndAliasIsNotUnknown(): void {
 });
 
 test(function flagBooleanTrueAnyDoubleHyphenArgumentIsNotUnknown(): void {
-  const unknown = [];
-  function unknownFn(arg): boolean {
+  const unknown: unknown[] = [];
+  function unknownFn(arg: unknown): boolean {
     unknown.push(arg);
     return false;
   }
@@ -40,8 +40,8 @@ test(function flagBooleanTrueAnyDoubleHyphenArgumentIsNotUnknown(): void {
 });
 
 test(function stringAndAliasIsNotUnkown(): void {
-  const unknown = [];
-  function unknownFn(arg): boolean {
+  const unknown: unknown[] = [];
+  function unknownFn(arg: unknown): boolean {
     unknown.push(arg);
     return false;
   }
@@ -59,8 +59,8 @@ test(function stringAndAliasIsNotUnkown(): void {
 });
 
 test(function defaultAndAliasIsNotUnknown(): void {
-  const unknown = [];
-  function unknownFn(arg): boolean {
+  const unknown: unknown[] = [];
+  function unknownFn(arg: unknown): boolean {
     unknown.push(arg);
     return false;
   }
@@ -78,8 +78,8 @@ test(function defaultAndAliasIsNotUnknown(): void {
 });
 
 test(function valueFollowingDoubleHyphenIsNotUnknown(): void {
-  const unknown = [];
-  function unknownFn(arg): boolean {
+  const unknown: unknown[] = [];
+  function unknownFn(arg: unknown): boolean {
     unknown.push(arg);
     return false;
   }
diff --git a/fs/copy.ts b/fs/copy.ts
index d02ca290f056..b51106e3ab6b 100644
--- a/fs/copy.ts
+++ b/fs/copy.ts
@@ -22,7 +22,7 @@ async function ensureValidCopy(
   options: CopyOptions,
   isCopyFolder: boolean = false
 ): Promise<Deno.FileInfo> {
-  let destStat: Deno.FileInfo;
+  let destStat: Deno.FileInfo | null;
 
   destStat = await Deno.lstat(dest).catch(
     (): Promise<null> => Promise.resolve(null)
@@ -39,7 +39,7 @@ async function ensureValidCopy(
     }
   }
 
-  return destStat;
+  return destStat!;
 }
 
 function ensureValidCopySync(
@@ -48,7 +48,7 @@ function ensureValidCopySync(
   options: CopyOptions,
   isCopyFolder: boolean = false
 ): Deno.FileInfo {
-  let destStat: Deno.FileInfo;
+  let destStat: Deno.FileInfo | null;
 
   try {
     destStat = Deno.lstatSync(dest);
@@ -56,8 +56,8 @@ function ensureValidCopySync(
     // ignore error
   }
 
-  if (destStat) {
-    if (isCopyFolder && !destStat.isDirectory()) {
+  if (destStat!) {
+    if (isCopyFolder && !destStat!.isDirectory()) {
       throw new Error(
         `Cannot overwrite non-directory '${dest}' with directory '${src}'.`
       );
@@ -67,7 +67,7 @@ function ensureValidCopySync(
     }
   }
 
-  return destStat;
+  return destStat!;
 }
 
 /* copy file to dest */
@@ -80,7 +80,7 @@ async function copyFile(
   await Deno.copyFile(src, dest);
   if (options.preserveTimestamps) {
     const statInfo = await Deno.stat(src);
-    await Deno.utime(dest, statInfo.accessed, statInfo.modified);
+    await Deno.utime(dest, statInfo.accessed!, statInfo.modified!);
   }
 }
 /* copy file to dest synchronously */
@@ -89,7 +89,7 @@ function copyFileSync(src: string, dest: string, options: CopyOptions): void {
   Deno.copyFileSync(src, dest);
   if (options.preserveTimestamps) {
     const statInfo = Deno.statSync(src);
-    Deno.utimeSync(dest, statInfo.accessed, statInfo.modified);
+    Deno.utimeSync(dest, statInfo.accessed!, statInfo.modified!);
   }
 }
 
@@ -105,7 +105,7 @@ async function copySymLink(
   await Deno.symlink(originSrcFilePath, dest, type);
   if (options.preserveTimestamps) {
     const statInfo = await Deno.lstat(src);
-    await Deno.utime(dest, statInfo.accessed, statInfo.modified);
+    await Deno.utime(dest, statInfo.accessed!, statInfo.modified!);
   }
 }
 
@@ -121,7 +121,7 @@ function copySymlinkSync(
   Deno.symlinkSync(originSrcFilePath, dest, type);
   if (options.preserveTimestamps) {
     const statInfo = Deno.lstatSync(src);
-    Deno.utimeSync(dest, statInfo.accessed, statInfo.modified);
+    Deno.utimeSync(dest, statInfo.accessed!, statInfo.modified!);
   }
 }
 
@@ -139,13 +139,13 @@ async function copyDir(
 
   if (options.preserveTimestamps) {
     const srcStatInfo = await Deno.stat(src);
-    await Deno.utime(dest, srcStatInfo.accessed, srcStatInfo.modified);
+    await Deno.utime(dest, srcStatInfo.accessed!, srcStatInfo.modified!);
   }
 
   const files = await Deno.readDir(src);
 
   for (const file of files) {
-    const srcPath = path.join(src, file.name);
+    const srcPath = path.join(src, file.name!);
     const destPath = path.join(dest, path.basename(srcPath as string));
     if (file.isDirectory()) {
       await copyDir(srcPath, destPath, options);
@@ -167,13 +167,13 @@ function copyDirSync(src: string, dest: string, options: CopyOptions): void {
 
   if (options.preserveTimestamps) {
     const srcStatInfo = Deno.statSync(src);
-    Deno.utimeSync(dest, srcStatInfo.accessed, srcStatInfo.modified);
+    Deno.utimeSync(dest, srcStatInfo.accessed!, srcStatInfo.modified!);
   }
 
   const files = Deno.readDirSync(src);
 
   for (const file of files) {
-    const srcPath = path.join(src, file.name);
+    const srcPath = path.join(src, file.name!);
     const destPath = path.join(dest, path.basename(srcPath as string));
     if (file.isDirectory()) {
       copyDirSync(srcPath, destPath, options);
diff --git a/fs/globrex.ts b/fs/globrex.ts
index 439cea348860..56def02f8187 100644
--- a/fs/globrex.ts
+++ b/fs/globrex.ts
@@ -66,7 +66,7 @@ export function globrex(
 
   // Helper function to build string and segments
   function add(
-    str,
+    str: string,
     options: AddOptions = { split: false, last: false, only: "" }
   ): void {
     const { split, last, only } = options;
@@ -114,13 +114,13 @@ export function globrex(
     if (c === ")") {
       if (ext.length) {
         add(c);
-        let type = ext.pop();
+        let type: string | undefined = ext.pop();
         if (type === "@") {
           add("{1}");
         } else if (type === "!") {
           add("([^/]*)");
         } else {
-          add(type);
+          add(type as string);
         }
         continue;
       }
diff --git a/fs/globrex_test.ts b/fs/globrex_test.ts
index 34821b6203d5..c6732b9ea788 100644
--- a/fs/globrex_test.ts
+++ b/fs/globrex_test.ts
@@ -5,29 +5,45 @@
 import { test } from "../testing/mod.ts";
 import { assertEquals } from "../testing/asserts.ts";
 import { globrex, GlobrexResult } from "./globrex.ts";
+import { GlobOptions } from "./glob.ts";
 
 const isWin = Deno.build.os === "win";
 const t = { equal: assertEquals, is: assertEquals };
 
-function match(glob, strUnix, strWin?, opts = {}): boolean {
+function match(
+  glob: string,
+  strUnix: string,
+  strWin?: string | object,
+  opts = {}
+): boolean {
   if (typeof strWin === "object") {
     opts = strWin;
-    strWin = false;
+    strWin = "";
   }
   let res = globrex(glob, opts);
   return res.regex.test(isWin && strWin ? strWin : strUnix);
 }
 
-function matchRegex(t, pattern, ifUnix, ifWin, opts): GlobrexResult {
+function matchRegex(
+  pattern: string,
+  ifUnix: string,
+  ifWin: string,
+  opts: GlobOptions
+): GlobrexResult {
   const res = globrex(pattern, opts);
-  const { regex } = opts.filepath ? res.path : res;
+  const { regex } = opts.filepath ? res.path! : res;
   t.is(regex.toString(), isWin ? ifWin : ifUnix, "~> regex matches expectant");
   return res;
 }
 
-function matchSegments(t, pattern, ifUnix, ifWin, opts): GlobrexResult {
+function matchSegments(
+  pattern: string,
+  ifUnix: RegExp[],
+  ifWin: RegExp[],
+  opts: GlobOptions
+): GlobrexResult {
   const res = globrex(pattern, { filepath: true, ...opts });
-  const str = res.path.segments.join(" ");
+  const str = res.path!.segments.join(" ");
   const exp = (isWin ? ifWin : ifUnix).join(" ");
   t.is(str, exp);
   return res;
@@ -191,7 +207,7 @@ test({
     t.equal(match("f?o", "fooo", { extended: true }), false);
     t.equal(match("f?oo", "foo", { extended: true }), false);
 
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       t.equal(
         match("f?o", "foo", { extended: true, globstar, flags: "g" }),
         true
@@ -235,7 +251,7 @@ test({
     t.equal(match("fo[!tz]", "fot", { extended: true }), false);
     t.equal(match("fo[!tz]", "fob", { extended: true }), true);
 
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       t.equal(
         match("fo[oz]", "foo", { extended: true, globstar, flags: "g" }),
         true
@@ -321,7 +337,7 @@ test({
     t.equal(match("foo{bar,baaz}", "foobuzz", { extended: true }), false);
     t.equal(match("foo{bar,b*z}", "foobuzz", { extended: true }), true);
 
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       t.equal(
         match("foo{bar,baaz}", "foobaaz", {
           extended: true,
@@ -405,7 +421,7 @@ test({
       false
     );
 
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       t.equal(
         match(
           "http://?o[oz].b*z.com/{*.js,*.html}",
@@ -456,7 +472,7 @@ test({
 test({
   name: "globrex: standard globstar",
   fn(): void {
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       t.equal(
         match(
           "http://foo.com/**/{*.js,*.html}",
@@ -491,7 +507,7 @@ test({
 test({
   name: "globrex: remaining chars should match themself",
   fn(): void {
-    const tester = (globstar): void => {
+    const tester = (globstar: boolean): void => {
       const testExtStr = "\\/$^+.()=!|,.*";
       t.equal(match(testExtStr, testExtStr, { extended: true }), true);
       t.equal(
@@ -801,47 +817,43 @@ test({
 
     res = globrex("", opts);
     t.is(res.hasOwnProperty("path"), true);
-    t.is(res.path.hasOwnProperty("regex"), true);
-    t.is(res.path.hasOwnProperty("segments"), true);
-    t.is(Array.isArray(res.path.segments), true);
+    t.is(res.path!.hasOwnProperty("regex"), true);
+    t.is(res.path!.hasOwnProperty("segments"), true);
+    t.is(Array.isArray(res.path!.segments), true);
 
     pattern = "foo/bar/baz.js";
     res = matchRegex(
-      t,
       pattern,
       "/^foo\\/bar\\/baz\\.js$/",
       "/^foo\\\\+bar\\\\+baz\\.js$/",
       opts
     );
-    t.is(res.path.segments.length, 3);
+    t.is(res.path!.segments.length, 3);
 
     res = matchRegex(
-      t,
       "../foo/bar.js",
       "/^\\.\\.\\/foo\\/bar\\.js$/",
       "/^\\.\\.\\\\+foo\\\\+bar\\.js$/",
       opts
     );
-    t.is(res.path.segments.length, 3);
+    t.is(res.path!.segments.length, 3);
 
     res = matchRegex(
-      t,
       "*/bar.js",
       "/^.*\\/bar\\.js$/",
       "/^.*\\\\+bar\\.js$/",
       opts
     );
-    t.is(res.path.segments.length, 2);
+    t.is(res.path!.segments.length, 2);
 
     opts.globstar = true;
     res = matchRegex(
-      t,
       "**/bar.js",
       "/^((?:[^\\/]*(?:\\/|$))*)bar\\.js$/",
       "/^((?:[^\\\\]*(?:\\\\|$))*)bar\\.js$/",
       opts
     );
-    t.is(res.path.segments.length, 2);
+    t.is(res.path!.segments.length, 2);
   }
 });
 
@@ -854,42 +866,42 @@ test({
 
     unix = [/^foo$/, /^bar$/, /^([^\/]*)$/, /^baz\.(md|js|txt)$/];
     win = [/^foo$/, /^bar$/, /^([^\\]*)$/, /^baz\.(md|js|txt)$/];
-    matchSegments(t, "foo/bar/*/baz.{md,js,txt}", unix, win, {
+    matchSegments("foo/bar/*/baz.{md,js,txt}", unix, win, {
       ...opts,
       globstar: true
     });
 
     unix = [/^foo$/, /^.*$/, /^baz\.md$/];
     win = [/^foo$/, /^.*$/, /^baz\.md$/];
-    matchSegments(t, "foo/*/baz.md", unix, win, opts);
+    matchSegments("foo/*/baz.md", unix, win, opts);
 
     unix = [/^foo$/, /^.*$/, /^baz\.md$/];
     win = [/^foo$/, /^.*$/, /^baz\.md$/];
-    matchSegments(t, "foo/**/baz.md", unix, win, opts);
+    matchSegments("foo/**/baz.md", unix, win, opts);
 
     unix = [/^foo$/, /^((?:[^\/]*(?:\/|$))*)$/, /^baz\.md$/];
     win = [/^foo$/, /^((?:[^\\]*(?:\\|$))*)$/, /^baz\.md$/];
-    matchSegments(t, "foo/**/baz.md", unix, win, { ...opts, globstar: true });
+    matchSegments("foo/**/baz.md", unix, win, { ...opts, globstar: true });
 
     unix = [/^foo$/, /^.*$/, /^.*\.md$/];
     win = [/^foo$/, /^.*$/, /^.*\.md$/];
-    matchSegments(t, "foo/**/*.md", unix, win, opts);
+    matchSegments("foo/**/*.md", unix, win, opts);
 
     unix = [/^foo$/, /^((?:[^\/]*(?:\/|$))*)$/, /^([^\/]*)\.md$/];
     win = [/^foo$/, /^((?:[^\\]*(?:\\|$))*)$/, /^([^\\]*)\.md$/];
-    matchSegments(t, "foo/**/*.md", unix, win, { ...opts, globstar: true });
+    matchSegments("foo/**/*.md", unix, win, { ...opts, globstar: true });
 
     unix = [/^foo$/, /^:$/, /^b:az$/];
     win = [/^foo$/, /^:$/, /^b:az$/];
-    matchSegments(t, "foo/:/b:az", unix, win, opts);
+    matchSegments("foo/:/b:az", unix, win, opts);
 
     unix = [/^foo$/, /^baz\.md$/];
     win = [/^foo$/, /^baz\.md$/];
-    matchSegments(t, "foo///baz.md", unix, win, { ...opts, strict: true });
+    matchSegments("foo///baz.md", unix, win, { ...opts, strict: true });
 
     unix = [/^foo$/, /^baz\.md$/];
     win = [/^foo$/, /^baz\.md$/];
-    matchSegments(t, "foo///baz.md", unix, win, { ...opts, strict: false });
+    matchSegments("foo///baz.md", unix, win, { ...opts, strict: false });
   }
 });
 
diff --git a/fs/path/join_test.ts b/fs/path/join_test.ts
index d82a1b47109a..2c0f35e48260 100644
--- a/fs/path/join_test.ts
+++ b/fs/path/join_test.ts
@@ -108,18 +108,21 @@ const windowsJoinTests = [
 
 test(function join() {
   joinTests.forEach(function(p) {
-    const actual = path.posix.join.apply(null, p[0]);
+    const _p = p[0] as string[];
+    const actual = path.posix.join.apply(null, _p);
     assertEquals(actual, p[1]);
   });
 });
 
 test(function joinWin32() {
   joinTests.forEach(function(p) {
-    const actual = path.win32.join.apply(null, p[0]).replace(backslashRE, "/");
+    const _p = p[0] as string[];
+    const actual = path.win32.join.apply(null, _p).replace(backslashRE, "/");
     assertEquals(actual, p[1]);
   });
   windowsJoinTests.forEach(function(p) {
-    const actual = path.win32.join.apply(null, p[0]);
+    const _p = p[0] as string[];
+    const actual = path.win32.join.apply(null, _p);
     assertEquals(actual, p[1]);
   });
 });
diff --git a/fs/path/parse_format_test.ts b/fs/path/parse_format_test.ts
index 05327a13950c..8829891da5f7 100644
--- a/fs/path/parse_format_test.ts
+++ b/fs/path/parse_format_test.ts
@@ -79,8 +79,8 @@ const unixSpecialCaseFormatTests = [
   [{}, ""]
 ];
 
-function checkParseFormat(path, paths): void {
-  paths.forEach(function(p) {
+function checkParseFormat(path: any, paths: any): void {
+  paths.forEach(function(p: Record<string, unknown>[]) {
     const element = p[0];
     const output = path.parse(element);
     assertEquals(typeof output.root, "string");
@@ -95,8 +95,8 @@ function checkParseFormat(path, paths): void {
   });
 }
 
-function checkSpecialCaseParseFormat(path, testCases): void {
-  testCases.forEach(function(testCase) {
+function checkSpecialCaseParseFormat(path: any, testCases: any): void {
+  testCases.forEach(function(testCase: Record<string, unknown>[]) {
     const element = testCase[0];
     const expect = testCase[1];
     const output = path.parse(element);
@@ -106,7 +106,7 @@ function checkSpecialCaseParseFormat(path, testCases): void {
   });
 }
 
-function checkFormat(path, testCases): void {
+function checkFormat(path: any, testCases: unknown[][]): void {
   testCases.forEach(function(testCase) {
     assertEquals(path.format(testCase[0]), testCase[1]);
   });
diff --git a/fs/path/resolve_test.ts b/fs/path/resolve_test.ts
index 7ee9513c1405..606570aadb3e 100644
--- a/fs/path/resolve_test.ts
+++ b/fs/path/resolve_test.ts
@@ -37,14 +37,16 @@ const posixTests =
 
 test(function resolve() {
   posixTests.forEach(function(p) {
-    const actual = path.posix.resolve.apply(null, p[0]);
+    const _p = p[0] as string[];
+    const actual = path.posix.resolve.apply(null, _p);
     assertEquals(actual, p[1]);
   });
 });
 
 test(function resolveWin32() {
   windowsTests.forEach(function(p) {
-    const actual = path.win32.resolve.apply(null, p[0]);
+    const _p = p[0] as string[];
+    const actual = path.win32.resolve.apply(null, _p);
     assertEquals(actual, p[1]);
   });
 });
diff --git a/fs/path/utils.ts b/fs/path/utils.ts
index e50cb6d3c49e..7a4cd7299e07 100644
--- a/fs/path/utils.ts
+++ b/fs/path/utils.ts
@@ -49,7 +49,7 @@ export function normalizeString(
   let code: number;
   for (let i = 0, len = path.length; i <= len; ++i) {
     if (i < len) code = path.charCodeAt(i);
-    else if (isPathSeparator(code)) break;
+    else if (isPathSeparator(code!)) break;
     else code = CHAR_FORWARD_SLASH;
 
     if (isPathSeparator(code)) {
@@ -107,7 +107,7 @@ export function _format(
   sep: string,
   pathObject: FormatInputPathObject
 ): string {
-  const dir: string | null = pathObject.dir || pathObject.root;
+  const dir: string | undefined = pathObject.dir || pathObject.root;
   const base: string =
     pathObject.base || (pathObject.name || "") + (pathObject.ext || "");
   if (!dir) return base;
diff --git a/fs/path/win32.ts b/fs/path/win32.ts
index 6b906bc7b3e2..d42cc4885d99 100644
--- a/fs/path/win32.ts
+++ b/fs/path/win32.ts
@@ -171,7 +171,7 @@ export function normalize(path: string): string {
   const len = path.length;
   if (len === 0) return ".";
   let rootEnd = 0;
-  let device: string;
+  let device: string | undefined;
   let isAbsolute = false;
   const code = path.charCodeAt(0);
 
@@ -301,7 +301,7 @@ export function join(...paths: string[]): string {
   const pathsCount = paths.length;
   if (pathsCount === 0) return ".";
 
-  let joined: string;
+  let joined: string | undefined;
   let firstPart: string;
   for (let i = 0; i < pathsCount; ++i) {
     let path = paths[i];
@@ -329,6 +329,7 @@ export function join(...paths: string[]): string {
   //   path.join('//server', 'share') -> '\\\\server\\share\\')
   let needsReplace = true;
   let slashCount = 0;
+  firstPart = firstPart!;
   if (isPathSeparator(firstPart.charCodeAt(0))) {
     ++slashCount;
     const firstLen = firstPart.length;
diff --git a/fs/utils.ts b/fs/utils.ts
index 83390ad706ed..ab5bf2f0ef63 100644
--- a/fs/utils.ts
+++ b/fs/utils.ts
@@ -16,10 +16,14 @@ export function isSubdir(
   }
   const srcArray = src.split(sep);
   const destArray = dest.split(sep);
-
-  return srcArray.reduce((acc, current, i): boolean => {
-    return acc && destArray[i] === current;
-  }, true);
+  // see: https://github.com/Microsoft/TypeScript/issues/30821
+  return srcArray.reduce(
+    // @ts-ignore
+    (acc: true, current: string, i: number): boolean => {
+      return acc && destArray[i] === current;
+    },
+    true
+  );
 }
 
 export type PathType = "file" | "dir" | "symlink";
@@ -29,7 +33,7 @@ export type PathType = "file" | "dir" | "symlink";
  *
  * @param fileInfo A FileInfo describes a file and is returned by `stat`, `lstat`
  */
-export function getFileInfoType(fileInfo: Deno.FileInfo): PathType {
+export function getFileInfoType(fileInfo: Deno.FileInfo): PathType | undefined {
   return fileInfo.isFile()
     ? "file"
     : fileInfo.isDirectory()
diff --git a/fs/walk.ts b/fs/walk.ts
index 5cfd7e63dbef..4520fe1746d2 100644
--- a/fs/walk.ts
+++ b/fs/walk.ts
@@ -71,7 +71,7 @@ export async function* walk(
   root: string,
   options: WalkOptions = {}
 ): AsyncIterableIterator<WalkInfo> {
-  options.maxDepth -= 1;
+  options.maxDepth! -= 1;
   let ls: FileInfo[] = [];
   try {
     ls = await readDir(root);
@@ -90,14 +90,14 @@ export async function* walk(
       }
     }
 
-    const filename = join(root, info.name);
+    const filename = join(root, info.name!);
 
     if (info.isFile()) {
       if (include(filename, options)) {
         yield { filename, info };
       }
     } else {
-      if (!(options.maxDepth < 0)) {
+      if (!(options.maxDepth! < 0)) {
         yield* walk(filename, options);
       }
     }
@@ -109,7 +109,7 @@ export function* walkSync(
   root: string = ".",
   options: WalkOptions = {}
 ): IterableIterator<WalkInfo> {
-  options.maxDepth -= 1;
+  options.maxDepth! -= 1;
   let ls: FileInfo[] = [];
   try {
     ls = readDirSync(root);
@@ -127,14 +127,14 @@ export function* walkSync(
       }
     }
 
-    const filename = join(root, info.name);
+    const filename = join(root, info.name!);
 
     if (info.isFile()) {
       if (include(filename, options)) {
         yield { filename, info };
       }
     } else {
-      if (!(options.maxDepth < 0)) {
+      if (!(options.maxDepth! < 0)) {
         yield* walkSync(filename, options);
       }
     }
diff --git a/fs/walk_test.ts b/fs/walk_test.ts
index 20e105a52641..1111f9f00f63 100644
--- a/fs/walk_test.ts
+++ b/fs/walk_test.ts
@@ -5,7 +5,7 @@ import { test, TestFunction, runIfMain } from "../testing/mod.ts";
 import { assertEquals } from "../testing/asserts.ts";
 
 export async function testWalk(
-  setup: (string) => void | Promise<void>,
+  setup: (arg0: string) => void | Promise<void>,
   t: TestFunction
 ): Promise<void> {
   const name = t.name;
diff --git a/http/cookie.ts b/http/cookie.ts
index 900d2941be2e..026ed19848e0 100644
--- a/http/cookie.ts
+++ b/http/cookie.ts
@@ -45,8 +45,8 @@ function toString(cookie: Cookie): string {
   if (cookie.httpOnly) {
     out.push("HttpOnly");
   }
-  if (Number.isInteger(cookie.maxAge)) {
-    assert(cookie.maxAge > 0, "Max-Age must be an integer superior to 0");
+  if (Number.isInteger(cookie.maxAge!)) {
+    assert(cookie.maxAge! > 0, "Max-Age must be an integer superior to 0");
     out.push(`Max-Age=${cookie.maxAge}`);
   }
   if (cookie.domain) {
@@ -75,10 +75,10 @@ function toString(cookie: Cookie): string {
 export function getCookies(req: ServerRequest): Cookies {
   if (req.headers.has("Cookie")) {
     const out: Cookies = {};
-    const c = req.headers.get("Cookie").split(";");
+    const c = req.headers.get("Cookie")!.split(";");
     for (const kv of c) {
       const cookieVal = kv.split("=");
-      const key = cookieVal.shift().trim();
+      const key = cookieVal.shift()!.trim();
       out[key] = cookieVal.join("=");
     }
     return out;
diff --git a/http/cookie_test.ts b/http/cookie_test.ts
index 4b4e2759d469..3d2e98afc57e 100644
--- a/http/cookie_test.ts
+++ b/http/cookie_test.ts
@@ -38,7 +38,7 @@ test({
     let res: Response = {};
     delCookie(res, "deno");
     assertEquals(
-      res.headers.get("Set-Cookie"),
+      res.headers!.get("Set-Cookie"),
       "deno=; Expires=Thus, 01 Jan 1970 00:00:00 GMT"
     );
   }
diff --git a/http/file_server_test.ts b/http/file_server_test.ts
index 1e2d86c4da1c..201524e7b0fb 100644
--- a/http/file_server_test.ts
+++ b/http/file_server_test.ts
@@ -6,7 +6,8 @@ import { assert, assertEquals } from "../testing/asserts.ts";
 import { BufReader, EOF } from "../io/bufio.ts";
 import { TextProtoReader } from "../textproto/mod.ts";
 
-let fileServer;
+let fileServer: Deno.Process;
+
 async function startFileServer(): Promise<void> {
   fileServer = run({
     args: [
@@ -21,14 +22,14 @@ async function startFileServer(): Promise<void> {
     stdout: "piped"
   });
   // Once fileServer is ready it will write to its stdout.
-  const r = new TextProtoReader(new BufReader(fileServer.stdout));
+  const r = new TextProtoReader(new BufReader(fileServer.stdout!));
   const s = await r.readLine();
   assert(s !== EOF && s.includes("server listening"));
 }
 
 function killFileServer(): void {
   fileServer.close();
-  fileServer.stdout.close();
+  fileServer.stdout!.close();
 }
 
 test(async function serveFile(): Promise<void> {
diff --git a/http/racing_server_test.ts b/http/racing_server_test.ts
index f98072c16fa7..5433665019dc 100644
--- a/http/racing_server_test.ts
+++ b/http/racing_server_test.ts
@@ -5,20 +5,20 @@ import { assert, assertEquals } from "../testing/asserts.ts";
 import { BufReader, EOF } from "../io/bufio.ts";
 import { TextProtoReader } from "../textproto/mod.ts";
 
-let server;
+let server: Deno.Process;
 async function startServer(): Promise<void> {
   server = run({
     args: ["deno", "run", "-A", "http/racing_server.ts"],
     stdout: "piped"
   });
   // Once fileServer is ready it will write to its stdout.
-  const r = new TextProtoReader(new BufReader(server.stdout));
+  const r = new TextProtoReader(new BufReader(server.stdout!));
   const s = await r.readLine();
   assert(s !== EOF && s.includes("Racing server listening..."));
 }
 function killServer(): void {
   server.close();
-  server.stdout.close();
+  server.stdout!.close();
 }
 
 let input = `GET / HTTP/1.1
diff --git a/http/server.ts b/http/server.ts
index 5ea52a0b8d9e..5c56a2ec2bf2 100644
--- a/http/server.ts
+++ b/http/server.ts
@@ -73,11 +73,10 @@ export async function writeResponse(w: Writer, r: Response): Promise<void> {
   let out = `HTTP/${protoMajor}.${protoMinor} ${statusCode} ${statusText}\r\n`;
 
   setContentLength(r);
+  const headers = r.headers!;
 
-  if (r.headers) {
-    for (const [key, value] of r.headers) {
-      out += `${key}: ${value}\r\n`;
-    }
+  for (const [key, value] of headers!) {
+    out += `${key}: ${value}\r\n`;
   }
   out += "\r\n";
 
@@ -88,8 +87,8 @@ export async function writeResponse(w: Writer, r: Response): Promise<void> {
   if (r.body instanceof Uint8Array) {
     const n = await writer.write(r.body);
     assert(n === r.body.byteLength);
-  } else if (r.headers.has("content-length")) {
-    const bodyLength = parseInt(r.headers.get("content-length"));
+  } else if (headers.has("content-length")) {
+    const bodyLength = parseInt(headers.get("content-length")!);
     const n = await copy(writer, r.body);
     assert(n === bodyLength);
   } else {
@@ -99,19 +98,19 @@ export async function writeResponse(w: Writer, r: Response): Promise<void> {
 }
 
 export class ServerRequest {
-  url: string;
-  method: string;
-  proto: string;
-  protoMinor: number;
-  protoMajor: number;
-  headers: Headers;
-  r: BufReader;
-  w: BufWriter;
+  url!: string;
+  method!: string;
+  proto!: string;
+  protoMinor!: number;
+  protoMajor!: number;
+  headers!: Headers;
+  r!: BufReader;
+  w!: BufWriter;
   done: Deferred<void> = deferred();
 
   public async *bodyStream(): AsyncIterableIterator<Uint8Array> {
     if (this.headers.has("content-length")) {
-      const len = +this.headers.get("content-length");
+      const len = +this.headers.get("content-length")!;
       if (Number.isNaN(len)) {
         return new Uint8Array(0);
       }
@@ -128,7 +127,7 @@ export class ServerRequest {
     } else {
       if (this.headers.has("transfer-encoding")) {
         const transferEncodings = this.headers
-          .get("transfer-encoding")
+          .get("transfer-encoding")!
           .split(",")
           .map((e): string => e.trim().toLowerCase());
         if (transferEncodings.includes("chunked")) {
@@ -334,14 +333,14 @@ export class Server implements AsyncIterable<ServerRequest> {
 
       // Wait for the request to be processed before we accept a new request on
       // this connection.
-      await req.done;
+      await req!.done;
     }
 
-    if (req === EOF) {
+    if (req! === EOF) {
       // The connection was gracefully closed.
     } else if (err) {
       // An error was thrown while parsing request headers.
-      await writeResponse(req.w, {
+      await writeResponse(req!.w, {
         status: 400,
         body: new TextEncoder().encode(`${err.message}\r\n\r\n`)
       });
diff --git a/http/server_test.ts b/http/server_test.ts
index e24ab0be9f49..2a6163d91426 100644
--- a/http/server_test.ts
+++ b/http/server_test.ts
@@ -336,65 +336,64 @@ malformedHeader
 // https://github.com/golang/go/blob/go1.12.5/src/net/http/request_test.go#L377-L443
 // TODO(zekth) fix tests
 test(async function testReadRequestError(): Promise<void> {
-  const testCases = {
-    0: {
+  const testCases = [
+    {
       in: "GET / HTTP/1.1\r\nheader: foo\r\n\r\n",
       headers: [{ key: "header", value: "foo" }]
     },
-    1: {
+    {
       in: "GET / HTTP/1.1\r\nheader:foo\r\n",
       err: UnexpectedEOFError
     },
-    2: { in: "", err: EOF },
-    3: {
+    { in: "", err: EOF },
+    {
       in: "HEAD / HTTP/1.1\r\nContent-Length:4\r\n\r\n",
       err: "http: method cannot contain a Content-Length"
     },
-    4: {
+    {
       in: "HEAD / HTTP/1.1\r\n\r\n",
       headers: []
     },
     // Multiple Content-Length values should either be
     // deduplicated if same or reject otherwise
     // See Issue 16490.
-    5: {
+    {
       in:
         "POST / HTTP/1.1\r\nContent-Length: 10\r\nContent-Length: 0\r\n\r\nGopher hey\r\n",
       err: "cannot contain multiple Content-Length headers"
     },
-    6: {
+    {
       in:
         "POST / HTTP/1.1\r\nContent-Length: 10\r\nContent-Length: 6\r\n\r\nGopher\r\n",
       err: "cannot contain multiple Content-Length headers"
     },
-    7: {
+    {
       in:
         "PUT / HTTP/1.1\r\nContent-Length: 6 \r\nContent-Length: 6\r\nContent-Length:6\r\n\r\nGopher\r\n",
       headers: [{ key: "Content-Length", value: "6" }]
     },
-    8: {
+    {
       in: "PUT / HTTP/1.1\r\nContent-Length: 1\r\nContent-Length: 6 \r\n\r\n",
       err: "cannot contain multiple Content-Length headers"
     },
     // Setting an empty header is swallowed by textproto
     // see: readMIMEHeader()
-    // 9: {
+    // {
     //   in: "POST / HTTP/1.1\r\nContent-Length:\r\nContent-Length: 3\r\n\r\n",
     //   err: "cannot contain multiple Content-Length headers"
     // },
-    10: {
+    {
       in: "HEAD / HTTP/1.1\r\nContent-Length:0\r\nContent-Length: 0\r\n\r\n",
       headers: [{ key: "Content-Length", value: "0" }]
     },
-    11: {
+    {
       in:
         "POST / HTTP/1.1\r\nContent-Length:0\r\ntransfer-encoding: chunked\r\n\r\n",
       headers: [],
       err: "http: Transfer-Encoding and Content-Length cannot be send together"
     }
-  };
-  for (const p in testCases) {
-    const test = testCases[p];
+  ];
+  for (const test of testCases) {
     const reader = new BufReader(new StringReader(test.in));
     let err;
     let req;
@@ -408,12 +407,12 @@ test(async function testReadRequestError(): Promise<void> {
     } else if (typeof test.err === "string") {
       assertEquals(err.message, test.err);
     } else if (test.err) {
-      assert(err instanceof test.err);
+      assert(err instanceof (test.err as typeof UnexpectedEOFError));
     } else {
       assertEquals(err, undefined);
       assertNotEquals(req, EOF);
-      for (const h of test.headers) {
-        assertEquals(req.headers.get(h.key), h.value);
+      for (const h of test.headers!) {
+        assertEquals((req! as ServerRequest).headers.get(h.key), h.value);
       }
     }
   }
diff --git a/io/bufio.ts b/io/bufio.ts
index 815c94eed8cd..99b5638161f0 100644
--- a/io/bufio.ts
+++ b/io/bufio.ts
@@ -40,8 +40,8 @@ export interface ReadLineResult {
 
 /** BufReader implements buffering for a Reader object. */
 export class BufReader implements Reader {
-  private buf: Uint8Array;
-  private rd: Reader; // Reader provided by caller.
+  private buf!: Uint8Array;
+  private rd!: Reader; // Reader provided by caller.
   private r = 0; // buf read position.
   private w = 0; // buf write position.
   private eof = false;
@@ -342,7 +342,7 @@ export class BufReader implements Reader {
       try {
         await this._fill();
       } catch (err) {
-        err.partial = slice;
+        err.partial = slice!;
         throw err;
       }
     }
@@ -439,7 +439,7 @@ export class BufWriter implements Writer {
     if (this.err !== null) throw this.err;
     if (this.n === 0) return;
 
-    let n: number;
+    let n = 0;
     try {
       n = await this.wr.write(this.buf.subarray(0, this.n));
     } catch (e) {
@@ -479,7 +479,7 @@ export class BufWriter implements Writer {
     if (p.length === 0) return 0;
 
     let nn = 0;
-    let n: number;
+    let n = 0;
     while (p.byteLength > this.available()) {
       if (this.buffered() === 0) {
         // Large write, empty buffer.
diff --git a/io/writers.ts b/io/writers.ts
index 86901872892d..de070d48a35d 100644
--- a/io/writers.ts
+++ b/io/writers.ts
@@ -6,6 +6,7 @@ import { decode, encode } from "../strings/mod.ts";
 export class StringWriter implements Writer {
   private chunks: Uint8Array[] = [];
   private byteLength: number = 0;
+  private cache: string | undefined;
 
   constructor(private base: string = "") {
     const c = encode(base);
@@ -16,12 +17,10 @@ export class StringWriter implements Writer {
   async write(p: Uint8Array): Promise<number> {
     this.chunks.push(p);
     this.byteLength += p.byteLength;
-    this.cache = null;
+    this.cache = undefined;
     return p.byteLength;
   }
 
-  private cache: string;
-
   toString(): string {
     if (this.cache) {
       return this.cache;
@@ -33,6 +32,6 @@ export class StringWriter implements Writer {
       offs += chunk.byteLength;
     }
     this.cache = decode(buf);
-    return this.cache;
+    return this.cache!;
   }
 }
diff --git a/log/handlers.ts b/log/handlers.ts
index f13a858e95f7..1f68e9794e09 100644
--- a/log/handlers.ts
+++ b/log/handlers.ts
@@ -40,14 +40,14 @@ export class BaseHandler {
     return this.formatter.replace(
       /{(\S+)}/g,
       (match, p1): string => {
-        const value = logRecord[p1];
+        const value = logRecord[p1 as keyof LogRecord];
 
         // do not interpolate missing values
         if (!value) {
           return match;
         }
 
-        return value;
+        return String(value);
       }
     );
   }
@@ -87,7 +87,7 @@ export class ConsoleHandler extends BaseHandler {
 }
 
 export abstract class WriterHandler extends BaseHandler {
-  protected _writer: Writer;
+  protected _writer!: Writer;
   private _encoder = new TextEncoder();
 
   log(msg: string): void {
@@ -100,7 +100,7 @@ interface FileHandlerOptions extends HandlerOptions {
 }
 
 export class FileHandler extends WriterHandler {
-  private _file: File;
+  private _file!: File;
   private _filename: string;
 
   constructor(levelName: string, options: FileHandlerOptions) {
diff --git a/log/levels.ts b/log/levels.ts
index 3fc86616186e..20cafb205729 100644
--- a/log/levels.ts
+++ b/log/levels.ts
@@ -1,5 +1,5 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-export const LogLevel = {
+export const LogLevel: Record<string, number> = {
   NOTSET: 0,
   DEBUG: 10,
   INFO: 20,
@@ -8,15 +8,6 @@ export const LogLevel = {
   CRITICAL: 50
 };
 
-const byName = {
-  NOTSET: LogLevel.NOTSET,
-  DEBUG: LogLevel.DEBUG,
-  INFO: LogLevel.INFO,
-  WARNING: LogLevel.WARNING,
-  ERROR: LogLevel.ERROR,
-  CRITICAL: LogLevel.CRITICAL
-};
-
 const byLevel = {
   [LogLevel.NOTSET]: "NOTSET",
   [LogLevel.DEBUG]: "DEBUG",
@@ -27,7 +18,7 @@ const byLevel = {
 };
 
 export function getLevelByName(name: string): number {
-  return byName[name];
+  return LogLevel[name];
 }
 
 export function getLevelName(level: number): string {
diff --git a/log/logger_test.ts b/log/logger_test.ts
index eb4e7673df93..2aca71d1ca9d 100644
--- a/log/logger_test.ts
+++ b/log/logger_test.ts
@@ -10,7 +10,7 @@ class TestHandler extends BaseHandler {
   public records: LogRecord[] = [];
 
   handle(record: LogRecord): void {
-    this.records.push({ ...record, datetime: null });
+    this.records.push({ ...record });
     super.handle(record);
   }
 
@@ -38,33 +38,29 @@ test(function customHandler(): void {
 
   logger.debug("foo", 1, 2);
 
-  assertEquals(handler.records, [
-    {
-      msg: "foo",
-      args: [1, 2],
-      datetime: null,
-      level: LogLevel.DEBUG,
-      levelName: "DEBUG"
-    }
-  ]);
+  const record = handler.records[0];
+  assertEquals(record.msg, "foo");
+  assertEquals(record.args, [1, 2]);
+  assertEquals(record.level, LogLevel.DEBUG);
+  assertEquals(record.levelName, "DEBUG");
 
   assertEquals(handler.messages, ["DEBUG foo"]);
 });
 
 test(function logFunctions(): void {
-  let handler: TestHandler;
-
-  const doLog = (level: string): void => {
-    handler = new TestHandler(level);
+  const doLog = (level: string): TestHandler => {
+    const handler = new TestHandler(level);
     let logger = new Logger(level, [handler]);
     logger.debug("foo");
     logger.info("bar");
     logger.warning("baz");
     logger.error("boo");
     logger.critical("doo");
+    return handler;
   };
 
-  doLog("DEBUG");
+  let handler: TestHandler;
+  handler = doLog("DEBUG");
 
   assertEquals(handler.messages, [
     "DEBUG foo",
@@ -74,7 +70,7 @@ test(function logFunctions(): void {
     "CRITICAL doo"
   ]);
 
-  doLog("INFO");
+  handler = doLog("INFO");
 
   assertEquals(handler.messages, [
     "INFO bar",
@@ -83,15 +79,15 @@ test(function logFunctions(): void {
     "CRITICAL doo"
   ]);
 
-  doLog("WARNING");
+  handler = doLog("WARNING");
 
   assertEquals(handler.messages, ["WARNING baz", "ERROR boo", "CRITICAL doo"]);
 
-  doLog("ERROR");
+  handler = doLog("ERROR");
 
   assertEquals(handler.messages, ["ERROR boo", "CRITICAL doo"]);
 
-  doLog("CRITICAL");
+  handler = doLog("CRITICAL");
 
   assertEquals(handler.messages, ["CRITICAL doo"]);
 });
diff --git a/log/mod.ts b/log/mod.ts
index 9149e860444f..d294cdc07377 100644
--- a/log/mod.ts
+++ b/log/mod.ts
@@ -50,7 +50,7 @@ export const handlers = {
 
 export function getLogger(name?: string): Logger {
   if (!name) {
-    return state.loggers.get("default");
+    return state.loggers.get("default")!;
   }
 
   if (!state.loggers.has(name)) {
@@ -59,7 +59,7 @@ export function getLogger(name?: string): Logger {
     return logger;
   }
 
-  return state.loggers.get(name);
+  return state.loggers.get(name)!;
 }
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -78,10 +78,6 @@ export const error = (msg: string, ...args: any[]): void =>
 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): Promise<void> {
   state.config = {
     handlers: { ...DEFAULT_CONFIG.handlers, ...config.handlers },
@@ -118,7 +114,7 @@ export async function setup(config: LogConfig): Promise<void> {
     handlerNames.forEach(
       (handlerName): void => {
         if (state.handlers.has(handlerName)) {
-          handlers.push(state.handlers.get(handlerName));
+          handlers.push(state.handlers.get(handlerName)!);
         }
       }
     );
diff --git a/log/test.ts b/log/test.ts
index 710de182f2ec..c42c7ed38cf6 100644
--- a/log/test.ts
+++ b/log/test.ts
@@ -16,7 +16,8 @@ class TestHandler extends log.handlers.BaseHandler {
 }
 
 test(async function defaultHandlers(): Promise<void> {
-  const loggers = {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const loggers: { [key: string]: (msg: string, ...args: any[]) => void } = {
     DEBUG: log.debug,
     INFO: log.info,
     WARNING: log.warning,
@@ -29,9 +30,8 @@ test(async function defaultHandlers(): Promise<void> {
       continue;
     }
 
-    const level = LogLevel[levelName];
     const logger = loggers[levelName];
-    const handler = new TestHandler(level);
+    const handler = new TestHandler(levelName);
 
     await log.setup({
       handlers: {
diff --git a/mime/multipart.ts b/mime/multipart.ts
index 580b81dc3138..461563a4e69d 100644
--- a/mime/multipart.ts
+++ b/mime/multipart.ts
@@ -177,20 +177,20 @@ class PartReader implements Reader, Closer {
 
   close(): void {}
 
-  private contentDisposition: string;
-  private contentDispositionParams: { [key: string]: string };
+  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(";");
+    const params: { [key: string]: string } = {};
+    const comps = cd!.split(";");
     this.contentDisposition = comps[0];
     comps
       .slice(1)
-      .map((v): string => v.trim())
+      .map((v: string): string => v.trim())
       .map(
-        (kv): void => {
+        (kv: string): void => {
           const [k, v] = kv.split("=");
           if (v) {
             const s = v.charAt(0);
@@ -292,7 +292,7 @@ export class MultipartReader {
           file.close();
           formFile = {
             filename: p.fileName,
-            type: p.headers.get("content-type"),
+            type: p.headers.get("content-type")!,
             tempfile: filepath,
             size
           };
@@ -302,20 +302,20 @@ export class MultipartReader {
       } else {
         formFile = {
           filename: p.fileName,
-          type: p.headers.get("content-type"),
+          type: p.headers.get("content-type")!,
           content: buf.bytes(),
           size: buf.length
         };
         maxMemory -= n;
         maxValueBytes -= n;
       }
-      result[p.formName] = formFile;
+      result[p.formName] = formFile!;
     }
     return result;
   }
 
-  private currentPart: PartReader;
-  private partsRead: number;
+  private currentPart: PartReader | undefined;
+  private partsRead: number = 0;
 
   private async nextPart(): Promise<PartReader | EOF> {
     if (this.currentPart) {
@@ -437,7 +437,7 @@ export class MultipartWriter {
     return this._boundary;
   }
 
-  private lastPart: PartWriter;
+  private lastPart: PartWriter | undefined;
   private bufWriter: BufWriter;
   private isClosed: boolean = false;
 
diff --git a/mime/multipart_test.ts b/mime/multipart_test.ts
index ed033ad9accd..a2fc960b7136 100644
--- a/mime/multipart_test.ts
+++ b/mime/multipart_test.ts
@@ -142,6 +142,7 @@ test(async function multipartMultipartWriter3(): Promise<void> {
   );
   await assertThrowsAsync(
     async (): Promise<void> => {
+      // @ts-ignore
       await mw.writeFile("bar", "file", null);
     },
     Error,
@@ -197,7 +198,7 @@ test(async function multipartMultipartReader2(): Promise<void> {
     assertEquals(form["bar"], "bar");
     const file = form["file"] as FormFile;
     assertEquals(file.type, "application/octet-stream");
-    const f = await open(file.tempfile);
+    const f = await open(file.tempfile!);
     const w = new StringWriter();
     await copy(w, f);
     const json = JSON.parse(w.toString());
@@ -205,7 +206,7 @@ test(async function multipartMultipartReader2(): Promise<void> {
     f.close();
   } finally {
     const file = form["file"] as FormFile;
-    await remove(file.tempfile);
+    await remove(file.tempfile!);
   }
 });
 
diff --git a/prettier/main_test.ts b/prettier/main_test.ts
index 86d6a75a50f4..bbd7f4bc1499 100644
--- a/prettier/main_test.ts
+++ b/prettier/main_test.ts
@@ -13,7 +13,7 @@ async function run(
 ): Promise<{ stdout: string; code: number | undefined }> {
   const p = xrun({ args, stdout: "piped" });
 
-  const stdout = decoder.decode(await readAll(p.stdout));
+  const stdout = decoder.decode(await readAll(p.stdout!));
   const { code } = await p.status();
 
   return { stdout, code };
diff --git a/prettier/testdata/opts/0.ts b/prettier/testdata/opts/0.ts
index fb85014a5e9e..400e7ee56ed2 100644
--- a/prettier/testdata/opts/0.ts
+++ b/prettier/testdata/opts/0.ts
@@ -1,2 +1,6 @@
-console.log(0)
-console.log([function foo() {}, function baz() {}, a => {}])
+console.log(0);
+console.log([
+    function foo() {},
+    function baz() {},
+    a => {}
+]);
diff --git a/prettier/util.ts b/prettier/util.ts
index 1c5513c8d037..b8f79005db00 100644
--- a/prettier/util.ts
+++ b/prettier/util.ts
@@ -2,7 +2,7 @@
 const { build, run } = Deno;
 
 // Runs a command in cross-platform way
-export function xrun(opts): Deno.Process {
+export function xrun(opts: Deno.RunOptions): Deno.Process {
   return run({
     ...opts,
     args: build.os === "win" ? ["cmd.exe", "/c", ...opts.args] : opts.args
diff --git a/testing/asserts.ts b/testing/asserts.ts
index a2583c22a983..cbdaf3a944d9 100644
--- a/testing/asserts.ts
+++ b/testing/asserts.ts
@@ -183,7 +183,7 @@ export function assertArrayContains(
   expected: unknown[],
   msg?: string
 ): void {
-  let missing = [];
+  let missing: unknown[] = [];
   for (let i = 0; i < expected.length; i++) {
     let found = false;
     for (let j = 0; j < actual.length; j++) {
diff --git a/testing/format.ts b/testing/format.ts
index 8ed066eb05a6..af8391b2308f 100644
--- a/testing/format.ts
+++ b/testing/format.ts
@@ -525,16 +525,10 @@ const getConfig = (options: Options): Config => ({
  */
 // 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): unknown => {
-      const opt = options[k];
-      if (typeof opt === "undefined") {
-        return { ...acc, [k]: DEFAULT_OPTIONS[k] };
-      }
-      return { ...acc, [k]: opt };
-    },
-    {}
-  ) as Options;
+  const opts: Options = {
+    ...DEFAULT_OPTIONS,
+    ...options
+  };
   const basicResult = printBasicValue(val, opts);
   if (basicResult !== null) {
     return basicResult;
diff --git a/testing/mod.ts b/testing/mod.ts
index fab8145e82fb..4ec241c206d1 100644
--- a/testing/mod.ts
+++ b/testing/mod.ts
@@ -54,7 +54,7 @@ interface TestStats {
 
 interface TestResult {
   name: string;
-  error: Error;
+  error?: Error;
   ok: boolean;
   printed: boolean;
 }
@@ -68,7 +68,7 @@ function createTestResults(tests: TestDefinition[]): TestResults {
   return tests.reduce(
     (acc: TestResults, { name }: TestDefinition, i: number): TestResults => {
       acc.keys.set(name, i);
-      acc.cases.set(i, { name, printed: false, ok: false, error: null });
+      acc.cases.set(i, { name, printed: false, ok: false, error: undefined });
       return acc;
     },
     { cases: new Map(), keys: new Map() }
@@ -114,11 +114,11 @@ function printResults(
 }
 
 function previousPrinted(name: string, results: TestResults): boolean {
-  const curIndex: number = results.keys.get(name);
+  const curIndex: number = results.keys.get(name)!;
   if (curIndex === 0) {
     return true;
   }
-  return results.cases.get(curIndex - 1).printed;
+  return results.cases.get(curIndex - 1)!.printed;
 }
 
 async function createTestCase(
@@ -127,7 +127,7 @@ async function createTestCase(
   exitOnFail: boolean,
   { fn, name }: TestDefinition
 ): Promise<void> {
-  const result: TestResult = results.cases.get(results.keys.get(name));
+  const result: TestResult = results.cases.get(results.keys.get(name)!)!;
   try {
     await fn();
     stats.passed++;
diff --git a/testing/pretty.ts b/testing/pretty.ts
index b2ab79d70220..f471bd6fd162 100644
--- a/testing/pretty.ts
+++ b/testing/pretty.ts
@@ -38,7 +38,7 @@ function createSign(diffType: DiffType): string {
 }
 
 function buildMessage(diffResult: ReadonlyArray<DiffResult<string>>): string[] {
-  const messages = [];
+  const messages: string[] = [];
   messages.push("");
   messages.push("");
   messages.push(
diff --git a/textproto/mod.ts b/textproto/mod.ts
index 66f303905e49..6c4e42e518b3 100644
--- a/textproto/mod.ts
+++ b/textproto/mod.ts
@@ -82,7 +82,7 @@ export class TextProtoReader {
       throw new UnexpectedEOFError();
     } else if (buf[0] == charCode(" ") || buf[0] == charCode("\t")) {
       throw new ProtocolError(
-        `malformed MIME header initial line: ${str(line)}`
+        `malformed MIME header initial line: ${str(line!)}`
       );
     }
 
@@ -140,7 +140,7 @@ export class TextProtoReader {
       const { line: l, more } = r;
 
       // Avoid the copy if the first call produced a full line.
-      if (!line && !more) {
+      if (!line! && !more) {
         // TODO(ry):
         // This skipSpace() is definitely misplaced, but I don't know where it
         // comes from nor how to fix it.
@@ -150,6 +150,7 @@ export class TextProtoReader {
         return l;
       }
 
+      // @ts-ignore
       line = append(line, l);
       if (!more) {
         break;
diff --git a/textproto/reader_test.ts b/textproto/reader_test.ts
index bd0d39fd339e..dfe918282078 100644
--- a/textproto/reader_test.ts
+++ b/textproto/reader_test.ts
@@ -83,7 +83,7 @@ test({
 test({
   name: "[textproto] Reader : Large MIME Header",
   async fn(): Promise<void> {
-    const data = [];
+    const data: string[] = [];
     // Go test is 16*1024. But seems it can't handle more
     for (let i = 0; i < 1024; i++) {
       data.push("x");
diff --git a/tsconfig.json b/tsconfig.json
index f943386c2837..f438205b013d 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,4 +1,5 @@
 {
+  "extends": "tsconfig.test.json",
   "compilerOptions": {
     "allowJs": true,
     "baseUrl": ".",
@@ -7,7 +8,6 @@
     "noEmit": true,
     "pretty": true,
     "resolveJsonModule": true,
-    "strict": true,
     "target": "esnext",
     "lib": ["esnext"]
   },
diff --git a/tsconfig.test.json b/tsconfig.test.json
new file mode 100644
index 000000000000..aee0ec940fcd
--- /dev/null
+++ b/tsconfig.test.json
@@ -0,0 +1,5 @@
+{
+  "compilerOptions": {
+    "strict": true
+  }
+}
diff --git a/util/async.ts b/util/async.ts
index 15edb1b42daf..b737c4cc8907 100644
--- a/util/async.ts
+++ b/util/async.ts
@@ -25,7 +25,7 @@ export function deferred<T>(): Deferred<T> {
       methods = { resolve, reject };
     }
   );
-  return Object.assign(promise, methods) as Deferred<T>;
+  return Object.assign(promise, methods)! as Deferred<T>;
 }
 
 interface TaggedYieldedValue<T> {
diff --git a/util/deep_assign.ts b/util/deep_assign.ts
index 77bf5e9b368a..1dfc00a5be5c 100644
--- a/util/deep_assign.ts
+++ b/util/deep_assign.ts
@@ -1,12 +1,15 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
-export function deepAssign(target: object, ...sources: object[]): object {
+export function deepAssign(
+  target: Record<string, unknown>,
+  ...sources: object[]
+): object | undefined {
   for (let i = 0; i < sources.length; i++) {
     const source = sources[i];
     if (!source || typeof source !== `object`) {
       return;
     }
     Object.entries(source).forEach(
-      ([key, value]): void => {
+      ([key, value]: [string, unknown]): void => {
         if (value instanceof Date) {
           target[key] = new Date(value);
           return;
@@ -22,7 +25,7 @@ export function deepAssign(target: object, ...sources: object[]): object {
         if (typeof target[key] !== `object` || !target[key]) {
           target[key] = {};
         }
-        deepAssign(target[key], value);
+        deepAssign(target[key] as Record<string, unknown>, value!);
       }
     );
   }
diff --git a/ws/mod.ts b/ws/mod.ts
index 7d8200dfcd95..2ae6cabda3e9 100644
--- a/ws/mod.ts
+++ b/ws/mod.ts
@@ -31,19 +31,25 @@ export interface WebSocketCloseEvent {
   reason?: string;
 }
 
-export function isWebSocketCloseEvent(a): a is WebSocketCloseEvent {
-  return a && typeof a["code"] === "number";
+export function isWebSocketCloseEvent(
+  a: WebSocketEvent
+): a is WebSocketCloseEvent {
+  return typeof a === "object" && a.hasOwnProperty("code");
 }
 
 export type WebSocketPingEvent = ["ping", Uint8Array];
 
-export function isWebSocketPingEvent(a): a is WebSocketPingEvent {
+export function isWebSocketPingEvent(
+  a: WebSocketEvent
+): a is WebSocketPingEvent {
   return Array.isArray(a) && a[0] === "ping" && a[1] instanceof Uint8Array;
 }
 
 export type WebSocketPongEvent = ["pong", Uint8Array];
 
-export function isWebSocketPongEvent(a): a is WebSocketPongEvent {
+export function isWebSocketPongEvent(
+  a: WebSocketEvent
+): a is WebSocketPongEvent {
   return Array.isArray(a) && a[0] === "pong" && a[1] instanceof Uint8Array;
 }
 
@@ -436,6 +442,8 @@ async function handshake(
   if (!m) {
     throw new Error("ws: invalid status line: " + statusLine);
   }
+
+  // @ts-ignore
   const { version, statusCode } = m.groups;
   if (version !== "HTTP/1.1" || statusCode !== "101") {
     throw new Error(
diff --git a/ws/sha1.ts b/ws/sha1.ts
index d01df21adf75..dc8ba680c977 100644
--- a/ws/sha1.ts
+++ b/ws/sha1.ts
@@ -77,11 +77,11 @@ export class Sha1 {
 
       if (notString) {
         for (i = start; index < length && i < 64; ++index) {
-          blocks[i >> 2] |= message[index] << SHIFT[i++ & 3];
+          blocks[i >> 2] |= (message[index] as number) << SHIFT[i++ & 3];
         }
       } else {
         for (i = start; index < length && i < 64; ++index) {
-          code = message.charCodeAt(index);
+          code = (message as string).charCodeAt(index);
           if (code < 0x80) {
             blocks[i >> 2] |= code << SHIFT[i++ & 3];
           } else if (code < 0x800) {
@@ -94,7 +94,8 @@ export class Sha1 {
           } else {
             code =
               0x10000 +
-              (((code & 0x3ff) << 10) | (message.charCodeAt(++index) & 0x3ff));
+              (((code & 0x3ff) << 10) |
+                ((message as string).charCodeAt(++index) & 0x3ff));
             blocks[i >> 2] |= (0xf0 | (code >> 18)) << SHIFT[i++ & 3];
             blocks[i >> 2] |= (0x80 | ((code >> 12) & 0x3f)) << SHIFT[i++ & 3];
             blocks[i >> 2] |= (0x80 | ((code >> 6) & 0x3f)) << SHIFT[i++ & 3];
diff --git a/ws/test.ts b/ws/test.ts
index bac82453dcb6..e14af1d559bd 100644
--- a/ws/test.ts
+++ b/ws/test.ts
@@ -166,20 +166,31 @@ test(function wsAcceptable(): void {
   );
 });
 
-const invalidHeaders = [
-  { "sec-websocket-key": "aaa" },
-  { upgrade: "websocket" },
-  { upgrade: "invalid", "sec-websocket-key": "aaa" },
-  { upgrade: "websocket", "sec-websocket-ky": "" }
-];
-
 test(function wsAcceptableInvalid(): void {
-  for (const pat of invalidHeaders) {
-    const ret = acceptable({
-      headers: new Headers(pat)
-    });
-    assertEquals(ret, false);
-  }
+  assertEquals(
+    acceptable({
+      headers: new Headers({ "sec-websocket-key": "aaa" })
+    }),
+    false
+  );
+  assertEquals(
+    acceptable({
+      headers: new Headers({ upgrade: "websocket" })
+    }),
+    false
+  );
+  assertEquals(
+    acceptable({
+      headers: new Headers({ upgrade: "invalid", "sec-websocket-key": "aaa" })
+    }),
+    false
+  );
+  assertEquals(
+    acceptable({
+      headers: new Headers({ upgrade: "websocket", "sec-websocket-ky": "" })
+    }),
+    false
+  );
 });
 
 test(async function wsWriteReadMaskedFrame(): Promise<void> {