title: Js compress by uglify-js give error Unexpected token: name ($)
body: <p>I am compress my js files by <strong><code>uglify-js</code></strong> 
     I want to compress Bootstrap 4 js file but it give me Error like this.
     Can I compress all js files by only <code>uglify-js</code> <strong>Or</strong> how can i do it.</p>
  
  <pre><code>    Parse error at src\bootstrap\alert.js:1,7
      import $ from 'jquery'
             ^
      ERROR: Unexpected token: name ($)
  </code></pre>
  
  <p>my <code>package.json</code> </p>
  
  <pre><code>"devDependencies": {
      "node-sass": "^4.6.1",
      "nodemon": "^1.12.1"
    },
    "dependencies": {
      "autoprefixer": "^7.1.6",
      "jquery": "^3.2.1",
      "postcss-cli": "^4.1.1",
      "uglify-js": "^3.1.9"
    }
  </code></pre>