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  <title>Lodash _.sumBy — missing fields</title>
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  <h1>_.sumBy — missing fields, null, NaN</h1>
  <pre id="out"></pre>
  <script src="https://cdn.jsdelivr.net/npm/lodash@4.17.21/lodash.min.js"></script>
  <script>
    (function () {
      var invoices = [
        { id: 1, total: 100 },
        { id: 2 },
        { id: 3, total: 50 }
      ];
      var sparse = [{ n: 1 }, { n: null }, {}];
      var lines = [
        "Missing property is SKIPPED (not NaN):",
        "  _.sumBy(invoices, 'total')                 => " + _.sumBy(invoices, "total"),
        "",
        "null in property coerces to 0:",
        "  _.sumBy([{n:1},{n:null},{n:3}], 'n')        => " + _.sumBy([{n:1},{n:null},{n:3}], "n"),
        "",
        "NaN poisons the result:",
        "  _.sumBy([{n:1},{n:NaN},{n:3}], 'n')         => " + _.sumBy([{n:1},{n:NaN},{n:3}], "n"),
        "",
        "Sparse rows (mix of missing + null):",
        "  _.sumBy(sparse, 'n')                        => " + _.sumBy(sparse, "n"),
        "  _.sumBy(sparse, r => r.n ?? 0)              => " + _.sumBy(sparse, function (r) { return r.n != null ? r.n : 0; }),
        "  _.sumBy(sparse, r => r.n || 0)              => " + _.sumBy(sparse, function (r) { return r.n || 0; }),
        "",
        "Friendly empty/null/undefined guard:",
        "  _.sumBy([], 'n')                            => " + _.sumBy([], "n"),
        "  _.sumBy(null, 'n')                          => " + _.sumBy(null, "n"),
        "  _.sumBy(undefined, 'n')                     => " + _.sumBy(undefined, "n")
      ];
      document.getElementById("out").textContent = lines.join("\n");
    })();
  </script>
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Lodash _.sumBy — missing fields, null, NaN (Try It)