检查属性名是否为数组索引

Question

我想为一个数组分配一些属性，但前提是它们是数组索引。否则，某些实现可能会将底层结构切换到哈希表，而我不想这样。

例如，这些是数组索引："0"、"1"、"2"、"3"、"4"、"4294967294"

但这些不是："abcd"、"0.1"、"-0"、"-1"、" 2"、"1e3"、"4294967295"

有没有一种简单的方法来测试一个字符串是否是数组索引？

- Oriol

Answer 1

在ECMAScript 5中，数组索引定义如下：Array indices：

如果一个属性名 P（以字符串形式）满足以下两个条件，则它是一个 array index：若ToUint32(P)不等于2³²−1，且ToString(ToUint32(P))等于P。

(在ECMAScript 2015中的定义口径不同但应该是等效的)

那么代码应该是:

function isArrayIndex(str) {
  return (str >>> 0) + '' === str && str < 4294967295
}

逐步地，

• ToUint32(P) can be done by shifting 0 bits with the unsigned right shift operator

  P >>> 0
  

• ToString(ToUint32(P)) can be done by concatenating the empty string with the addition operator.

  (P >>> 0) + ''
  

• ToString(ToUint32(P)) is equal to P can be checked with the strict equals operator.

  (P >>> 0) + '' === P
  

  Note this will also ensure that P really was in the form of a String value.

• ToUint32(P) is not equal to 2³²−1 can be checked with the strict does-not-equal operator

  (P >>> 0) !== 4294967295
  

  But once we know ToString(ToUint32(P)) is equal to P, one of the following should be enough:

  P !== "4294967295"
  P < 4294967295
  

- Oriol