Array.prototype.keys()

¥Syntax

keys()

¥Parameters

没有任何。

¥None.

¥Return value

新的 可迭代的迭代器对象。

¥A new iterable iterator object.

¥Description

当在 稀疏数组 上使用时，keys() 方法会迭代空槽，就好像它们具有值 undefined 一样。

¥When used on sparse arrays, the keys() method iterates empty slots as if they have the value undefined.

keys() 方法是 generic。它只期望 this 值具有 length 属性和整数键控属性。

¥The keys() method is generic. It only expects the this value to have a length property and integer-keyed properties.

¥Examples

¥Using keys() on sparse arrays

与 Object.keys() 不同，Object.keys() 只包含数组中实际存在的键，keys() 迭代器不会忽略代表缺失属性的漏洞。

¥Unlike Object.keys(), which only includes keys that actually exist in the array, the keys() iterator doesn't ignore holes representing missing properties.

const arr = ["a", , "c"];
const sparseKeys = Object.keys(arr);
const denseKeys = [...arr.keys()];
console.log(sparseKeys); // ['0', '2']
console.log(denseKeys); // [0, 1, 2]

¥Calling keys() on non-array objects

keys() 方法读取 this 的 length 属性，然后生成 0 到 length - 1 之间的所有整数索引。实际上没有发生索引访问。

¥The keys() method reads the length property of this and then yields all integer indices between 0 and length - 1. No index access actually happens.

const arrayLike = {
  length: 3,
};
for (const entry of Array.prototype.keys.call(arrayLike)) {
  console.log(entry);
}
// 0
// 1
// 2

¥See also

• core-js 中 Array.prototype.keys 的 Polyfill
• 索引集合 指南
• Array
• Array.prototype.entries()
• Array.prototype.values()
• Array.prototype[@@iterator]()
• TypedArray.prototype.keys()
• 迭代协议