javascript - arr.forEach 未产生预期结果

Question

这个问题已经有答案了:

forEach on a 'new Array' isn't doing what I expect (4 个回答)

forEach on array of undefined created by Array constructor (5 个回答)

已关闭 5 年前。

我正在尝试填充使用 Array(5) 创建的数组每个元素的索引:

let arr = Array(5)

arr.forEach((item, index) => arr[index] = index)

console.log(arr)

// expected output:
// [0, 1, 2, 3, 4]

// instead I get 
// [
//   undefined,
//   undefined,
//   undefined,
//   undefined,
//   undefined
// ]

我哪里错了？

Answer 1

您需要用函数 .fill() 填充它

该数组实际上是空的，因此 forEach 不会迭代。

new Array(arrayLength)

arrayLength

If the only argument passed to the Array constructor is an integer between 0 and 232-1 (inclusive), this returns a new JavaScript array with its length property set to that number (Note: this implies an array of arrayLength empty slots, not slots with actual undefined values). If the argument is any other number, a RangeError exception is thrown.

let arr = Array(5).fill();
arr.forEach((item, index) => { arr[index] = index })
console.log(arr);

Why does my code work when I change the first line to let arr = [...Array(5)]?

let arr = Array(5); 
Array.from(arr) // equals to [...arr];

.apply 将省略的元素“扩展”为适当的参数，结果最终类似于 Array(undefined, undefined, undefined, undefined, undefined) Reference