Question

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javascript in operator

Why does ("a" in ["a","b"]) yield false, and (1 in [1,2]) yield true ?
Is there a reason why "a" does not match the first element of that array and 1 does ?
Why won’t it work with strings ?

Answer 1

The in operator checks for the existence of properties by key, not by value. And your array of length 2 has an index "1" – arr["1"] is the value 2. For example, also 0 in ["a", "b"] is true. The behaviour does not depend on a string or a number being used.

You usually would use it on plain objects, not on arrays. Like "a" in {a:1} === true, or "b" in {a:1} === false.