title: Error "Uncaught ReferenceError: $ is not defined"
body: <p>I began to study the "javascript".</p>
  
  <p>in html source code</p>
  
  <pre><code>&lt;!Doctype html&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;meta charset="utf-8"&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;button id="test_button"&gt;Test&lt;/button&gt;
      &lt;p id="test"&gt;Hello World&lt;/p&gt;
  
      &lt;!-- script --&gt;
      &lt;script src="test.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>in javascript source code</p>
  
  <pre><code>"use strict";
  
  $(function() {
  document.getElementById("test_button").onclick = test_click;
  function test_click() {
      document.getElementById("test").innerHTML = "HI";
  }
  });
  </code></pre>
  
  <p>Where's wrong?</p>