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  <title>Lodash _.assignInWith — per-key rule</title>
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  <h1>_.assignInWith — per-key rule</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var target = { a: 1, b: 2 };
      var source1 = { b: 3, c: 4 };
      var source2 = { d: 5 };
      function customizer(objValue, srcValue, key) {
        if (key === "b") {
          return (objValue || 0) * (srcValue || 1);
        }
      }
      _.assignInWith(target, source1, source2, customizer);
      document.getElementById("out").textContent = JSON.stringify(target, null, 2);
    })();
  </script>
</body>
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Lodash _.assignInWith — per-key rule with undefined fallback (Try It)