Off By One Join

Joining an array of four gives three separators.

In 2026: Still works, and it trips people building repeated strings. join places the separator between elements, so an array of length 4 produces 3 of them. Array(4).join("pizza") gives three, and Array(n) is a common way people accidentally get n minus 1 copies.

Where it came from: join inserts the separator between elements, so N elements yield N-1 separators, per MDN's join page. MDN ↗

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document.write('<p><code>Array(4).join("\ud83c\udf55")</code> is <b>' + Array(4).join("\ud83c\udf55") + '</b> (three, not four)</p>');
document.write('<p><code>["a", "b", "c"].join(" - ")</code> is <b>' + ["a","b","c"].join(" - ") + '</b></p>');
document.write('<p>join goes between elements, so N elements give N minus 1 separators.</p>');
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