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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>isFunction — guard</title>
</head>
<body>
  <h1>Callback guard</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    function run(onReady) {
      if (_.isFunction(onReady)) {
        return onReady("ok");
      }
      return "skipped";
    }
    document.getElementById("out").textContent =
      run(function (msg) { return "got " + msg; }) + "\n" +
      run(null);
  </script>
</body>
</html>

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Lodash _.isFunction — callback guard (Try It)