javascript operator “in”

Question

I\'m used to python, so

a = [1,2,3]
1 in a # -> True
b = [\"1\", \"2\", \"3\", \"x\"]
\"x\" in b # -> True

Why is it that in JavaScript

Answer 1

1 is a valid vanilla numeric array index, x is not; in works with array indexes/object member names, not their values.