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Array.prototype.keys()

Array.prototype.keys() 方法用于获取一个新的 Iterator 对象，它包含数组中每个索引的键。

语法

语法：

arr.keys();

类型声明：

interface IteratorYieldResult<TYield> {
  done?: false;
  value: TYield;
}

interface IteratorReturnResult<TReturn> {
  done: true;
  value: TReturn;
}

type IteratorResult<T, TReturn = any> = IteratorYieldResult<T> | IteratorReturnResult<TReturn>;

interface Iterator<T, TReturn = any, TNext = undefined> {
  // NOTE: 'next' is defined using a tuple to ensure we report the correct assignability errors in all places.
  next(...args: [] | [TNext]): IteratorResult<T, TReturn>;
  return?(value?: TReturn): IteratorResult<T, TReturn>;
  throw?(e?: any): IteratorResult<T, TReturn>;
}

interface IterableIterator<T> extends Iterator<T> {
  [Symbol.iterator](): IterableIterator<T>;
}

interface Array<T> {
  keys(): IterableIterator<number>;
}

返回值：

返回 一个新的 Array Iterator 对象。

代码示例

let arr = ['a', 'b', 'c'];

let iterator = arr.keys();
// undefined

console.log(iterator);
// Array Iterator {}

console.log(iterator.next());
// Object {value: 0, done: false}

console.log(iterator.next());
// Object {value: 1, done: false}

console.log(iterator.next());
// Object {value: 2, done: false}

console.log(iterator.next());
// Object {value: undefined, done: true}

索引迭代器会包含那些没有对应元素的索引。

const arr = ['a', , 'c'];
const sparseKeys = Object.keys(arr);
const denseKeys = [...arr.keys()];

console.log(sparseKeys); // ['0', '2']

console.log(denseKeys); // [0, 1, 2]

参考资料