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  <title>Lodash _.merge - arrays and undefined</title>
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  <h1>_.merge - arrays by index; undefined skipped</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var positional = _.merge({}, { items: [1, 2, 3] }, { items: [10] });
      var concatenated = _.mergeWith(
        {},
        { items: [1, 2, 3] },
        { items: [10] },
        function (objValue, srcValue) {
          if (Array.isArray(objValue)) {
            return objValue.concat(srcValue);
          }
        }
      );
      var undefSkipped = _.merge(
        {},
        { name: "John", role: "admin" },
        { name: undefined, role: null }
      );
      var lines = [
        "positional (merge)     : " + JSON.stringify(positional),
        "concatenated (mergeWith): " + JSON.stringify(concatenated),
        "",
        "undefined skipped       : " + JSON.stringify(undefSkipped),
        "(name kept, role replaced by null)"
      ];
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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Lodash _.merge - arrays merge by index; undefined is skipped (Try It)