Javascript : separating array defined value that's undefined and array value that's not defined

Question

我现在如何检查该值是否定义为未定义，或者它是否真的未定义？
例如。

var a = [];
a[0] = undefined;

// a defined value that's undefined
typeof a[0] === "undefined";

// and a value that hasn't been defined at all
typeof a[1] === "undefined";

有没有办法把这两者分开？可以使用 for-in 循环遍历数组，但是有没有更简单的方法？

Answer 1

您可以检查索引是否在给定数组中:

0 in a // => true
1 in a // => false