debugger-examples/examples/optimized-away.html at master · firefox-devtools/debugger-examples · GitHub

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<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Optimized Away</title>
    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
    <style>
    body { padding-top: 2em; }
    </style>
  </head>
  <body>
    <div class="container">
      <button class="btn btn-primary" id="c">Click to Pause</button> and then:
      <ol>
        <li>select <strong>z</strong> in the <u>Call Stack</u></li>
        <li>look for <strong>x</strong> under the <strong>f</strong> group in the <u>Scopes</u> section</li>
      </ol>
    </div>
    <script>
      function f(x, y) {
        console.log(x);
        return function z() {
          return breakpoint(y);
        };
      }

      function breakpoint() {
        debugger;
      }

      var closure = f(10, "yess");

      function click() {
        closure();
      }
      document.getElementById("c").onclick = click;
    </script>

  </body>
</html>