lint gulpfile, add eslint config and ignore file for those that using eslint

.eslintignore

@@ -0,0 +1,2 @@
+js/**/*.js
+node_modules/**/*.js

.eslintrc.js

@@ -0,0 +1,39 @@
+module.exports = {
+  "root": true,
+  "parserOptions": {
+    "parser": "babel-eslint",
+    "ecmaVersion": 2017,
+    "sourceType": "module"
+  },
+  "env": {
+    "browser": true,
+    "node": true,
+    "es6": true
+  },
+  "globals": {
+    "window": true,
+    "location": true
+  },
+  "extends": [
+    "airbnb-base",
+  ],
+  // custom rules here
+  "rules": {
+    // don"t require .vue extension when importing
+    "import/extensions": ["error", "always", {
+      "js": "never",
+      "vue": "js",
+      "mjs": "never"
+    }],
+    "no-param-reassign": ["error", {
+      "props": true,
+      "ignorePropertyModificationsFor": [
+        "event", // for e.returnvalue
+        "response", // for Express responses
+        "item", // for item usually within each loops
+      ]
+    }],
+    // allow debugger during development
+    "no-debugger": process.env.NODE_ENV === "production" ? 2 : 0
+  }
+}

gulpfile.babel.js

@@ -1,4 +1,3 @@
-/* eslint-disable no-console */
 import gulp from 'gulp';
 import clean from 'gulp-clean';
 import sass from 'gulp-sass';
@@ -20,7 +19,7 @@ import notify from 'gulp-notify';
 import runSequence from 'run-sequence';
 import path from 'path';
 
-const log = console.log;
+const { log } = console;
 let production = false;
 let error = false;
 
@@ -127,7 +126,7 @@ gulp.task('scripts', ['clean-scripts'], () => {
       sourceMap: !production,
       dest: `${config.public}/js/script.min.js`,
     });
-    }).catch(err => {
+  }).catch((err) => {
     notification('Failed to compile scripts. 😱', 'error');
     log(err.stack);
     error = true;