Why does javascript’s “in” operator return true when testing if 0 exists in an array that doesn’t contain 0?

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It refers to the index or key, not the value. 0 and 1 are the valid indices for that array. There are also valid keys, including "length" and "toString". Try 2 in x. That will be false (since JavaScript arrays are 0-indexed).

See the MDN documentation.

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