Add eslint for TypeScript.

.eslintrc

@@ -13,6 +13,33 @@
     "mocha": true,
     "node": true
   },
+  "overrides": [
+    {
+      "files": [
+        "**/*.ts"
+      ],
+      "excludedFiles": [
+        "test/**/*.ts"
+      ],
+      "extends": [
+        "eslint:recommended",
+        "plugin:@typescript-eslint/eslint-recommended",
+        "plugin:@typescript-eslint/recommended"
+      ],
+      "parser": "@typescript-eslint/parser",
+      "plugins": [
+        "@typescript-eslint"
+      ],
+      "parserOptions": {
+        "ecmaVersion": 2020,
+        "sourceType": "module",
+        "ecmaFeatures": {
+          "jsx": false
+        },
+        "project": "./tsconfig.json"
+      }
+    }
+  ],
   "rules": {
     "eqeqeq": [
       "error",

lib/utils/addDetailedError.ts

@@ -1,4 +1,4 @@
-import { NightwatchError } from "./types";
+import {NightwatchError} from './types';
 
 /**
  * @method addDetailedError

lib/utils/browsername.ts

@@ -22,6 +22,6 @@ class BrowserName {
   get OPERA() {
     return 'opera';
   }
-};
+}
 
 export = new BrowserName();

lib/utils/createPromise.ts

@@ -1,11 +1,11 @@
-import { Deferred } from "./types";
+interface Deferred<T> {
+  promise: Promise<T>;
+  resolve: ((value: T | PromiseLike<T>) => void);
+  reject: ((reason?: unknown) => void);
+}
 
 export = function createPromise<T>(): Deferred<T> {
-  const deferred: Deferred<T> = {
-    resolve: null,
-    reject: null,
-    promise: null
-  };
+  const deferred = <Deferred<T>> {};
 
   deferred.promise = new Promise((resolve, reject) => {
     deferred.resolve = resolve;

lib/utils/types/index.ts

@@ -1,11 +1,5 @@
 export interface NightwatchError extends Error {
-    detailedErr: string;
-    link: string;
-    help: string[];
-}
-
-export interface Deferred<T> {
-    promise: Promise<T> | null;
-    resolve: ((value: T | PromiseLike<T>) => void) | null;
-    reject: ((reason?: any) => void) | null;
+  detailedErr: string;
+  link: string;
+  help: string[];
 }