Add eslint configuration and fix linting issues

.eslintrc.json

@@ -0,0 +1,47 @@
+{
+    "env": {
+        "node": true,
+        "es2020": true
+    },
+    "extends": "eslint:recommended",
+    "parserOptions": {
+        "ecmaVersion": "latest",
+        "sourceType": "commonjs"
+    },
+    "rules": {
+        "no-unused-vars": ["error", {
+            /* 
+            * The \^[A-Z][a-z]+\ pattern is intended to allow the importing of constructor functions (a.k.a classes)
+            * for the sole purpose of using them in JSDoc type annotations. Without this, eslint
+            * would complain, because it doesn't recognize usage in JSDoc comments as actual usage.
+            */ 
+            "varsIgnorePattern": "^[A-Z][a-z]+|^_",
+            "argsIgnorePattern": "^_",
+            "caughtErrorsIgnorePattern": "^_"
+        }]
+    },
+    "overrides": [
+        {
+            "files": ["src/**/*.js"],
+            "env": {
+                "browser": true
+            },
+            "parserOptions": {
+                "sourceType": "module"
+            },
+            "globals": {
+                "chrome": "readonly",
+                "WEBPACK_MODE": "readonly"
+            }
+        },
+        {
+            "files": ["tests/**/*.js"],
+            "env": {
+                "jest": true
+            },
+            "parserOptions": {
+                "sourceType": "commonjs"
+            }
+        }
+    ]
+}

globals.d.ts

@@ -0,0 +1 @@
+declare var WEBPACK_MODE: string;

jest.config.js

@@ -1,6 +1,7 @@
+// eslint-disable-next-line no-undef
 module.exports = {
     testMatch: ["<rootDir>/tests/**/*.test.js"],
-    // moduleNameMapper: {
-    //     "^src/(.*)": "<rootDir>/src/$1"
-    // }
-};
+    moduleNameMapper: {
+        "^src/(.*)": "<rootDir>/src/$1"
+    }
+};

jsconfig.json

@@ -4,10 +4,12 @@
         "noEmit": true,
         "target": "es6",
         "module": "commonjs",
-        "lib": ["es2016", "dom"],
-        "typeRoots": ["node_modules/@types"]
+        "lib": ["es2019", "dom"],
+        "typeRoots": ["node_modules/@types"],
+        "types": ["chrome"]
     },
     "include": [
-        "src/**/*"
+        "src/**/*",
+        "globals.d.ts"
     ]
 }