JavaScript中的in?

[skyover]

<script>
var strs = ['a','b','c','d'];
var str = 'a';
alert(str in strs);
</script>

怎么是false呢？

[skyover]

<script>
var strs = [1,2,3,4,5];
var str = 2;
alert(str in strs);
</script>

显示为true

<script>
var strs = [200,404,500];
var str = 200;
alert(str in strs);
</script>

显示为false

究意怎么回事。

[scoutlin]

你对JS的in理解错了