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  <title>Lodash _.subtract — undefined trap</title>
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  <h1>_.subtract — the undefined sign-flip trap</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    function safeSubtract(a, b) {
      return _.subtract(a == null ? 0 : a, b == null ? 0 : b);
    }
    (function () {
      var lines = [
        "_.subtract(undefined, undefined) => " + _.subtract(undefined, undefined),
        "_.subtract(undefined, 5)          => " + _.subtract(undefined, 5) + "   BUG (expected -5)",
        "_.subtract(5, undefined)          => " + _.subtract(5, undefined) + "   (consistent: 5 - 0 = 5)",
        "",
        "null coerces to 0 (correct math):",
        "  _.subtract(null, 5)              => " + _.subtract(null, 5),
        "  _.subtract(5, null)              => " + _.subtract(5, null),
        "  _.subtract(null, null)           => " + _.subtract(null, null),
        "",
        "Safe helper (a ?? 0 and b ?? 0):",
        "  safeSubtract(undefined, 5)       => " + safeSubtract(undefined, 5),
        "  safeSubtract(5, undefined)       => " + safeSubtract(5, undefined),
        "  safeSubtract(undefined, undefined) => " + safeSubtract(undefined, undefined)
      ];
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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Lodash _.subtract — the undefined sign-flip trap (Try It)