diff --git a/docs/api/dialog.md b/docs/api/dialog.md
index ac526fa50cd7..356621d748f5 100644
--- a/docs/api/dialog.md
+++ b/docs/api/dialog.md
@@ -115,7 +115,7 @@ dialog.showOpenDialogSync(mainWindow, {
 Returns `Promise<Object>` - Resolve with an object containing the following:
 
 * `canceled` Boolean - whether or not the dialog was canceled.
-* `filePaths` String[] (optional) - An array of file paths chosen by the user. If the dialog is cancelled this will be an empty array.
+* `filePaths` String[] - An array of file paths chosen by the user. If the dialog is cancelled this will be an empty array.
 * `bookmarks` String[] (optional) _macOS_ _mas_ - An array matching the `filePaths` array of base64 encoded strings which contains security scoped bookmark data. `securityScopedBookmarks` must be enabled for this to be populated.
 
 The `browserWindow` argument allows the dialog to attach itself to a parent window, making it modal.
diff --git a/filenames.auto.gni b/filenames.auto.gni
index c09c15a2e935..b6822d11515b 100644
--- a/filenames.auto.gni
+++ b/filenames.auto.gni
@@ -126,13 +126,13 @@ auto_filenames = {
     "lib/common/api/module-list.js",
     "lib/common/api/native-image.js",
     "lib/common/api/shell.js",
-    "lib/common/buffer-utils.js",
-    "lib/common/clipboard-utils.js",
+    "lib/common/buffer-utils.ts",
+    "lib/common/clipboard-utils.ts",
     "lib/common/crash-reporter.js",
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/common/is-promise.ts",
-    "lib/common/web-view-methods.js",
+    "lib/common/web-view-methods.ts",
     "lib/renderer/api/crash-reporter.js",
     "lib/renderer/api/desktop-capturer.ts",
     "lib/renderer/api/ipc-renderer.js",
@@ -166,7 +166,7 @@ auto_filenames = {
 
   isolated_bundle_deps = [
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/isolated_renderer/init.js",
     "lib/renderer/ipc-renderer-internal-utils.ts",
     "lib/renderer/ipc-renderer-internal.ts",
@@ -180,7 +180,7 @@ auto_filenames = {
 
   content_script_bundle_deps = [
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/content_script/init.js",
     "lib/renderer/chrome-api.ts",
     "lib/renderer/extensions/event.ts",
@@ -241,7 +241,7 @@ auto_filenames = {
     "lib/browser/crash-reporter-init.js",
     "lib/browser/default-menu.ts",
     "lib/browser/desktop-capturer.ts",
-    "lib/browser/devtools.js",
+    "lib/browser/devtools.ts",
     "lib/browser/guest-view-manager.js",
     "lib/browser/guest-window-manager.js",
     "lib/browser/init.ts",
@@ -257,16 +257,16 @@ auto_filenames = {
     "lib/common/api/module-list.js",
     "lib/common/api/native-image.js",
     "lib/common/api/shell.js",
-    "lib/common/buffer-utils.js",
-    "lib/common/clipboard-utils.js",
+    "lib/common/buffer-utils.ts",
+    "lib/common/clipboard-utils.ts",
     "lib/common/crash-reporter.js",
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/common/init.ts",
     "lib/common/is-promise.ts",
     "lib/common/parse-features-string.js",
     "lib/common/reset-search-paths.ts",
-    "lib/common/web-view-methods.js",
+    "lib/common/web-view-methods.ts",
     "lib/renderer/ipc-renderer-internal-utils.ts",
     "lib/renderer/ipc-renderer-internal.ts",
     "package.json",
@@ -282,15 +282,15 @@ auto_filenames = {
     "lib/common/api/module-list.js",
     "lib/common/api/native-image.js",
     "lib/common/api/shell.js",
-    "lib/common/buffer-utils.js",
-    "lib/common/clipboard-utils.js",
+    "lib/common/buffer-utils.ts",
+    "lib/common/clipboard-utils.ts",
     "lib/common/crash-reporter.js",
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/common/init.ts",
     "lib/common/is-promise.ts",
     "lib/common/reset-search-paths.ts",
-    "lib/common/web-view-methods.js",
+    "lib/common/web-view-methods.ts",
     "lib/renderer/api/crash-reporter.js",
     "lib/renderer/api/desktop-capturer.ts",
     "lib/renderer/api/exports/electron.js",
@@ -332,11 +332,11 @@ auto_filenames = {
     "lib/common/api/module-list.js",
     "lib/common/api/native-image.js",
     "lib/common/api/shell.js",
-    "lib/common/buffer-utils.js",
-    "lib/common/clipboard-utils.js",
+    "lib/common/buffer-utils.ts",
+    "lib/common/clipboard-utils.ts",
     "lib/common/crash-reporter.js",
     "lib/common/electron-binding-setup.ts",
-    "lib/common/error-utils.js",
+    "lib/common/error-utils.ts",
     "lib/common/init.ts",
     "lib/common/is-promise.ts",
     "lib/common/reset-search-paths.ts",
diff --git a/lib/browser/devtools.js b/lib/browser/devtools.ts
similarity index 75%
rename from lib/browser/devtools.js
rename to lib/browser/devtools.ts
index 028ac2ffb83b..1a7b3caf764e 100644
--- a/lib/browser/devtools.js
+++ b/lib/browser/devtools.ts
@@ -1,14 +1,12 @@
-'use strict'
-
-const { dialog, Menu } = require('electron')
-const fs = require('fs')
-const url = require('url')
+import { dialog, Menu } from 'electron'
+import * as fs from 'fs'
+import * as url from 'url'
 
 const ipcMainUtils = require('@electron/internal/browser/ipc-main-internal-utils')
 
-const convertToMenuTemplate = function (items, handler) {
+const convertToMenuTemplate = function (items: ContextMenuItem[], handler: (id: number) => void) {
   return items.map(function (item) {
-    const transformed = item.type === 'subMenu' ? {
+    const transformed: Electron.MenuItemConstructorOptions = item.type === 'subMenu' ? {
       type: 'submenu',
       label: item.label,
       enabled: item.enabled,
@@ -34,7 +32,7 @@ const convertToMenuTemplate = function (items, handler) {
   })
 }
 
-const getEditMenuItems = function () {
+const getEditMenuItems = function (): Electron.MenuItemConstructorOptions[] {
   return [
     { role: 'undo' },
     { role: 'redo' },
@@ -42,18 +40,18 @@ const getEditMenuItems = function () {
     { role: 'cut' },
     { role: 'copy' },
     { role: 'paste' },
-    { role: 'pasteAndMatchStyle' },
+    { role: 'pasteandmatchstyle' },
     { role: 'delete' },
-    { role: 'selectAll' }
+    { role: 'selectall' }
   ]
 }
 
-const isChromeDevTools = function (pageURL) {
+const isChromeDevTools = function (pageURL: string) {
   const { protocol } = url.parse(pageURL)
   return protocol === 'devtools:'
 }
 
-const assertChromeDevTools = function (contents, api) {
+const assertChromeDevTools = function (contents: Electron.WebContents, api: string) {
   const pageURL = contents._getURL()
   if (!isChromeDevTools(pageURL)) {
     console.error(`Blocked ${pageURL} from calling ${api}`)
@@ -61,7 +59,7 @@ const assertChromeDevTools = function (contents, api) {
   }
 }
 
-ipcMainUtils.handle('ELECTRON_INSPECTOR_CONTEXT_MENU', function (event, items, isEditMenu) {
+ipcMainUtils.handle('ELECTRON_INSPECTOR_CONTEXT_MENU', function (event: Electron.IpcMainEvent, items: ContextMenuItem[], isEditMenu: boolean) {
   return new Promise(resolve => {
     assertChromeDevTools(event.sender, 'window.InspectorFrontendHost.showContextMenuAtPoint()')
 
@@ -73,7 +71,7 @@ ipcMainUtils.handle('ELECTRON_INSPECTOR_CONTEXT_MENU', function (event, items, i
   })
 })
 
-ipcMainUtils.handle('ELECTRON_INSPECTOR_SELECT_FILE', async function (event) {
+ipcMainUtils.handle('ELECTRON_INSPECTOR_SELECT_FILE', async function (event: Electron.IpcMainEvent) {
   assertChromeDevTools(event.sender, 'window.UI.createFileSelectorElement()')
 
   const result = await dialog.showOpenDialog({})
@@ -85,7 +83,7 @@ ipcMainUtils.handle('ELECTRON_INSPECTOR_SELECT_FILE', async function (event) {
   return [path, data]
 })
 
-ipcMainUtils.handle('ELECTRON_INSPECTOR_CONFIRM', async function (event, message = '', title = '') {
+ipcMainUtils.handle('ELECTRON_INSPECTOR_CONFIRM', async function (event: Electron.IpcMainEvent, message: string = '', title: string = '') {
   assertChromeDevTools(event.sender, 'window.confirm()')
 
   const options = {
diff --git a/lib/common/buffer-utils.js b/lib/common/buffer-utils.ts
similarity index 54%
rename from lib/common/buffer-utils.js
rename to lib/common/buffer-utils.ts
index 8bfdf66ee390..01d0786b716f 100644
--- a/lib/common/buffer-utils.js
+++ b/lib/common/buffer-utils.ts
@@ -1,11 +1,6 @@
-'use strict'
+import { Buffer } from 'buffer'
 
-// Note: Don't use destructuring assignment for `Buffer`, or we'll hit a
-// browserify bug that makes the statement invalid, throwing an error in
-// sandboxed renderer.
-const Buffer = require('buffer').Buffer
-
-const typedArrays = {
+const typedArrays: Record<string, Function> = {
   Buffer,
   ArrayBuffer,
   Int8Array,
@@ -19,16 +14,18 @@ const typedArrays = {
   Float64Array
 }
 
-function getType (value) {
+type BufferLike = Buffer | ArrayBuffer | ArrayBufferView
+
+function getType (value: BufferLike) {
   for (const type of Object.keys(typedArrays)) {
     if (value instanceof typedArrays[type]) {
       return type
     }
   }
-  return null
+  throw new Error('Invalid buffer')
 }
 
-function getBuffer (value) {
+function getBuffer (value: BufferLike) {
   if (value instanceof Buffer) {
     return value
   } else if (value instanceof ArrayBuffer) {
@@ -38,19 +35,27 @@ function getBuffer (value) {
   }
 }
 
-exports.isBuffer = function (value) {
+export function isBuffer (value: BufferLike) {
   return ArrayBuffer.isView(value) || value instanceof ArrayBuffer
 }
 
-exports.bufferToMeta = function (value) {
+interface BufferMeta {
+  type: keyof typeof typedArrays;
+  data: Buffer;
+  length: number | undefined;
+}
+
+export function bufferToMeta (value: BufferLike): BufferMeta {
   return {
     type: getType(value),
     data: getBuffer(value),
-    length: value.length
+    // NB. We only use length when decoding Int8Array and friends.
+    // For other buffer-like types this is expected to be undefined.
+    length: (value as Buffer).length
   }
 }
 
-exports.metaToBuffer = function (value) {
+export function metaToBuffer (value: BufferMeta) {
   const constructor = typedArrays[value.type]
   const data = getBuffer(value.data)
 
@@ -59,7 +64,7 @@ exports.metaToBuffer = function (value) {
   } else if (constructor === ArrayBuffer) {
     return data.buffer
   } else if (constructor) {
-    return new constructor(data.buffer, data.byteOffset, value.length)
+    return new (constructor as any)(data.buffer, data.byteOffset, value.length)
   } else {
     return data
   }
diff --git a/lib/common/clipboard-utils.js b/lib/common/clipboard-utils.ts
similarity index 84%
rename from lib/common/clipboard-utils.js
rename to lib/common/clipboard-utils.ts
index f6e8be74be92..166bf74be562 100644
--- a/lib/common/clipboard-utils.js
+++ b/lib/common/clipboard-utils.ts
@@ -1,14 +1,12 @@
-'use strict'
-
 const { nativeImage, NativeImage } = process.electronBinding('native_image')
 
-const objectMap = function (source, mapper) {
+const objectMap = function (source: Object, mapper: (value: any) => any) {
   const sourceEntries = Object.entries(source)
   const targetEntries = sourceEntries.map(([key, val]) => [key, mapper(val)])
   return Object.fromEntries(targetEntries)
 }
 
-const serialize = function (value) {
+export function serialize (value: any): any {
   if (value instanceof NativeImage) {
     return {
       buffer: value.toBitmap(),
@@ -26,7 +24,7 @@ const serialize = function (value) {
   }
 }
 
-const deserialize = function (value) {
+export function deserialize (value: any): any {
   if (value && value.__ELECTRON_SERIALIZED_NativeImage__) {
     return nativeImage.createFromBitmap(value.buffer, value.size)
   } else if (Array.isArray(value)) {
@@ -39,8 +37,3 @@ const deserialize = function (value) {
     return value
   }
 }
-
-module.exports = {
-  serialize,
-  deserialize
-}
diff --git a/lib/common/error-utils.js b/lib/common/error-utils.ts
similarity index 75%
rename from lib/common/error-utils.js
rename to lib/common/error-utils.ts
index 26dd0185d3f4..b6c084918b22 100644
--- a/lib/common/error-utils.js
+++ b/lib/common/error-utils.ts
@@ -1,5 +1,3 @@
-'use strict'
-
 const constructors = new Map([
   [Error.name, Error],
   [EvalError.name, EvalError],
@@ -10,10 +8,10 @@ const constructors = new Map([
   [URIError.name, URIError]
 ])
 
-exports.deserialize = function (error) {
+export function deserialize (error: Electron.SerializedError): Electron.ErrorWithCause {
   if (error && error.__ELECTRON_SERIALIZED_ERROR__ && constructors.has(error.name)) {
     const constructor = constructors.get(error.name)
-    const deserializedError = new constructor(error.message)
+    const deserializedError = new constructor!(error.message) as Electron.ErrorWithCause
     deserializedError.stack = error.stack
     deserializedError.from = error.from
     deserializedError.cause = exports.deserialize(error.cause)
@@ -22,7 +20,7 @@ exports.deserialize = function (error) {
   return error
 }
 
-exports.serialize = function (error) {
+export function serialize (error: Electron.ErrorWithCause): Electron.SerializedError {
   if (error instanceof Error) {
     // Errors get lost, because: JSON.stringify(new Error('Message')) === {}
     // Take the serializable properties and construct a generic object
@@ -30,7 +28,7 @@ exports.serialize = function (error) {
       message: error.message,
       stack: error.stack,
       name: error.name,
-      from: process.type,
+      from: process.type as Electron.ProcessType,
       cause: exports.serialize(error.cause),
       __ELECTRON_SERIALIZED_ERROR__: true
     }
diff --git a/lib/common/web-view-methods.js b/lib/common/web-view-methods.ts
similarity index 93%
rename from lib/common/web-view-methods.js
rename to lib/common/web-view-methods.ts
index c81655cb438a..f3b6c3c0de31 100644
--- a/lib/common/web-view-methods.js
+++ b/lib/common/web-view-methods.ts
@@ -1,7 +1,5 @@
-'use strict'
-
 // Public-facing API methods.
-exports.syncMethods = new Set([
+export const syncMethods = new Set([
   'getURL',
   'getTitle',
   'isLoading',
@@ -52,7 +50,7 @@ exports.syncMethods = new Set([
   'setZoomLevel'
 ])
 
-exports.asyncMethods = new Set([
+export const asyncMethods = new Set([
   'loadURL',
   'capturePage',
   'executeJavaScript',
diff --git a/lib/renderer/inspector.ts b/lib/renderer/inspector.ts
index 9e06478f9b25..9739f24c5344 100644
--- a/lib/renderer/inspector.ts
+++ b/lib/renderer/inspector.ts
@@ -22,7 +22,7 @@ function completeURL (project: string, path: string) {
   return invokeSync('ELECTRON_INSPECTOR_CONFIRM', message, title) as boolean
 }
 
-const useEditMenuItems = function (x: number, y: number, items: any[]) {
+const useEditMenuItems = function (x: number, y: number, items: ContextMenuItem[]) {
   return items.length === 0 && document.elementsFromPoint(x, y).some(function (element) {
     return element.nodeName === 'INPUT' ||
       element.nodeName === 'TEXTAREA' ||
@@ -30,7 +30,7 @@ const useEditMenuItems = function (x: number, y: number, items: any[]) {
   })
 }
 
-const createMenu = function (x: number, y: number, items: any[]) {
+const createMenu = function (x: number, y: number, items: ContextMenuItem[]) {
   const isEditMenu = useEditMenuItems(x, y, items)
   invoke<number>('ELECTRON_INSPECTOR_CONTEXT_MENU', items, isEditMenu).then(id => {
     if (typeof id === 'number') {
diff --git a/tsconfig.json b/tsconfig.json
index 562d65f3f90a..0abc258da456 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -3,7 +3,7 @@
     "module": "commonjs",
     "target": "es2017",
     "lib": [
-      "es2017",
+      "es2019",
       "dom",
       "dom.iterable"
     ],
diff --git a/typings/internal-ambient.d.ts b/typings/internal-ambient.d.ts
index 8ba166b73746..5801ab94a3a9 100644
--- a/typings/internal-ambient.d.ts
+++ b/typings/internal-ambient.d.ts
@@ -54,11 +54,20 @@ declare module NodeJS  {
   }
 }
 
+interface ContextMenuItem {
+  id: number;
+  label: string;
+  type: 'normal' | 'separator' | 'subMenu' | 'checkbox';
+  checked: boolean;
+  enabled: boolean;
+  subItems: ContextMenuItem[];
+}
+
 declare interface Window {
   ELECTRON_DISABLE_SECURITY_WARNINGS?: boolean;
   ELECTRON_ENABLE_SECURITY_WARNINGS?: boolean;
   InspectorFrontendHost?: {
-    showContextMenuAtPoint: (x: number, y: number, items: any[]) => void
+    showContextMenuAtPoint: (x: number, y: number, items: ContextMenuItem[]) => void
   };
   DevToolsAPI?: {
     contextMenuItemSelected: (id: number) => void;
diff --git a/typings/internal-electron.d.ts b/typings/internal-electron.d.ts
index 94de2cfdfbc5..f2c48a154eae 100644
--- a/typings/internal-electron.d.ts
+++ b/typings/internal-electron.d.ts
@@ -19,6 +19,11 @@ declare namespace Electron {
     setAppPath(path: string | null): void;
   }
 
+  interface WebContents {
+    _getURL(): string;
+    getOwnerBrowserWindow(): Electron.BrowserWindow;
+  }
+
   interface SerializedError {
     message: string;
     stack?: string,
@@ -28,6 +33,11 @@ declare namespace Electron {
     __ELECTRON_SERIALIZED_ERROR__: true
   }
 
+  interface ErrorWithCause extends Error {
+    from?: string;
+    cause?: ErrorWithCause;
+  }
+
   interface InjectionBase {
     url: string;
     code: string