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  <title>Lodash _.defaults — fill gaps</title>
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<body>
  <h1>_.defaults — fill gaps</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var userSettings    = { username: "JohnDoe", theme: "light" };
      var defaultSettings = { theme: "dark", language: "en" };
      _.defaults(userSettings, defaultSettings);
      document.getElementById("out").textContent =
        JSON.stringify(userSettings, null, 2);
    })();
  </script>
</body>
</html>

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Lodash _.defaults — fill only the missing keys (Try It)