问题
Possible Duplicate:
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 ?
回答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.
来源：https://stackoverflow.com/questions/11808263/why-does-a-in-a-b-yield-false-and-1-in-1-2-yield-true