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  <title>Lodash _.merge - mutation behavior</title>
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  <h1>_.merge - destination is mutated</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var user = {
        name: "John",
        address: { city: "New York", country: "USA" }
      };
      var updates = {
        age: 35,
        address: { city: "San Francisco" }
      };
      var result = _.merge(user, updates);
      var fresh = _.merge({}, { x: 1 }, { y: 2 });
      var lines = [
        "result === user      : " + (result === user),
        "user.address.city    : " + user.address.city,
        "user.age             : " + user.age,
        "",
        "fresh                : " + JSON.stringify(fresh),
        "fresh === { x: 1 }   : false  (always a new object)"
      ];
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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Lodash _.merge - mutation matters (Try It)