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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>assignWith — own keys</title>
</head>
<body>
  <h1>Own keys only</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    function Foo() {
      this.a = 1;
    }
    Foo.prototype.b = 2;
    function customizer(objValue, srcValue) {
      return _.isUndefined(objValue) ? srcValue : objValue;
    }
    document.getElementById("out").textContent =
      "assignWith: " + JSON.stringify(_.assignWith({ a: 9 }, new Foo(), customizer)) + "\n" +
      "assignInWith: " + JSON.stringify(_.assignInWith({ a: 9 }, new Foo(), customizer));
  </script>
</body>
</html>

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Lodash _.assignWith — own keys (Try It)