HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Lodash _.mergeWith - undefined fallback</title>
</head>
<body>
  <h1>_.mergeWith - undefined fallback</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var left = {
        options: { theme: "light", pageSize: 20 },
        roles: ["reader"]
      };
      var right = {
        options: { pageSize: 50 },
        roles: ["admin"]
      };
      var safe = _.mergeWith({}, left, right, function (objValue, srcValue) {
        if (Array.isArray(objValue)) {
          return objValue.concat(srcValue);
        }
        return undefined;
      });
      var tooBroad = _.mergeWith({}, left, right, function (objValue, srcValue) {
        if (Array.isArray(objValue)) {
          return objValue.concat(srcValue);
        }
        return srcValue;
      });
      var lines = [
        "safe (undefined fallback):",
        JSON.stringify(safe, null, 2),
        "",
        "tooBroad (returns srcValue for everything):",
        JSON.stringify(tooBroad, null, 2),
        "",
        "Notice how tooBroad replaced options.theme instead of deep-merging it."
      ];
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
</body>
</html>

Lines: 0 | Characters: 0

Live Preview Auto-refresh on Run

Lodash _.mergeWith - undefined fallback rule (Try It)