Excerpt from my JavaScript console:
> 0 in [1, 2]
true
Why?
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Because “in” returns true if the specified property/index is available in the object. [1, 2] is an array, and has a object at the 0 index. Hence, 0 in [1, 2], and 1 in [1, 2]. But !(2 in [1, 2]).
Edit: For what you probably intended, David Dorward’s comment below is very useful. If you (somewhat perversely) want to stick with ‘in’, you could use an object literal
This should allow
1 in x && 2 in x && !(0 in x)etc. But really, just use indexOf.