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  <title>Lodash _.assignInWith — keep existing</title>
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  <h1>_.assignInWith — keep existing defined values</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var userPrefs = { theme: "dark", lang: undefined };
      var defaults  = { theme: "light", lang: "en", fontSize: 14 };
      function keepDefined(objValue, srcValue) {
        return objValue === undefined ? srcValue : objValue;
      }
      _.assignInWith(userPrefs, defaults, keepDefined);
      document.getElementById("out").textContent = JSON.stringify(userPrefs, null, 2);
    })();
  </script>
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Lodash _.assignInWith — keep existing defined values (Try It)