Add linting for ES Modules

Also fixes the linting issues and some feedback from the PR.

package.json

@@ -24,7 +24,9 @@
     "test": "run-s test-node test-headless test-webworker test-es-module",
     "check-dependencies": "dependency-check package.json --unused --no-dev --ignore-module coveralls --ignore-module @std/esm",
     "build": "node ./build.js",
-    "lint": "eslint .",
+    "lint": "run-p lint-js lint-mjs lint-markdown",
+    "lint-js": "eslint .",
+    "lint-mjs": "eslint --ext mjs --parser-options=sourceType:module test/es2015",
     "lint-markdown": "find docs -type f -name '*.md' ! -name 'changelog.md' | xargs markdownlint",
     "precommit": "lint-staged",
     "pretest-webworker": "npm run build",
@@ -35,6 +37,7 @@
   },
   "lint-staged": {
     "*.js": "eslint",
+    "*.mjs": "eslint --ext mjs --parser-options=sourceType:module",
     "docs/**/*.md": "markdownlint"
   },
   "dependencies": {

test/es2015/a-function-module.mjs

@@ -1 +1 @@
-export default function (){ return 42; }
+export default function () { return 42; }

test/es2015/a-module-with-default.mjs

@@ -1,3 +1,3 @@
 export default {
-    anExport(){ return 42; }
-}
+    anExport() { return 42; }
+};

test/es2015/a-module.mjs

@@ -1 +1 @@
-export function anExport(){ return 42; }
+export function anExport() { return 42; }

test/es2015/module-support-assessment-test.mjs

@@ -1,55 +1,57 @@
-"use strict";
+
 import referee from "referee";
 import sinon from "../../lib/sinon";
 import * as aModule from "./a-module";
 import aModuleWithDefaultExport from "./a-module-with-default";
+
+// Usually one would import the default module, but one can make a form of wrapper like this
+/* eslint-disable no-unused-vars */
 import functionModule, * as functionModuleAlternative from "./a-function-module";
 const {assert, refute} = referee;
 
-function createTestSuite(action){
+function createTestSuite(action) {
     var stub;
     var errorRegEx = /TypeError: ES Modules cannot be (stubbed|spied)/;
 
-    afterEach(function() {
-        stub && stub.restore && stub.restore();
-    });
+    describe("sinon." + action + "()", function () {
 
-    describe("sinon." + action + "()", function(){
-        describe("Modules with objects as their default export", function() {
-    afterEach(function() {
-        stub && stub.restore && stub.restore();
-    });
-            it("should NOT result in error", function() {
-                refute.exception(function() {
+        afterEach(function () {
+            if (stub && stub.restore) { stub.restore(); }
+        });
+
+        describe("Modules with objects as their default export", function () {
+
+            it("should NOT result in error", function () {
+                refute.exception(function () {
                     stub = sinon[action](aModuleWithDefaultExport, "anExport");
                 });
             });
 
-            it("should spy/stub an exported function", function() {
+            it("should spy/stub an exported function", function () {
                 stub = sinon[action](aModuleWithDefaultExport, "anExport");
                 aModuleWithDefaultExport.anExport();
                 aModuleWithDefaultExport.anExport();
                 assert(stub.callCount === 2);
             });
         });
 
-        describe("Modules without default export", function() {
-            it("should give a proper error message", function() {
-                assert.exception(function() {
+        describe("Modules without default export", function () {
+            it("should give a proper error message", function () {
+                assert.exception(function () {
                     sinon[action](aModule, "anExport");
                 }, errorRegEx);
             });
         });
 
-        describe("Modules that exports a function as their default export", function() {
-            it("should not be possible to spy/stub the default export using a wrapper for the exports", function() {
-                assert.exception(function() {
+        describe("Modules that exports a function as their default export", function () {
+            it("should not be possible to spy/stub the default export using a wrapper for the exports", function () {
+                assert.exception(function () {
                     stub = sinon[action](functionModuleAlternative, "anExport");
                 }, errorRegEx);
             });
         });
     });
 }
 
-createTestSuite('stub');
-createTestSuite('spy');
+createTestSuite("stub");
+createTestSuite("spy");