为什么我不能将新的Array（3）映射到新值的数组中？ [英] Why can't I map new Array(3) into an array of new values?

问题描述

我想了解背后的为什么：

var a = new Array(3);
var b = a.map(function () {
  return 'b';
});

结果

a： [,,]

b： [,,]

当我希望b导致 ['b'，'b'，'b'] 。

在进一步调查中，我发现如果我要做 a.push（'a' ），我有 [,,,'a'] 。

In further investigation, I discovered that if i were to do a.push('a'), I'd have [, , , 'a'].

在地图功能之后， b 将变为 [,,,'b' ] 。

And after the map function, b would become [, , , 'b'].

这里发生了什么？为什么这些分配的单元格的行为与初始化不同？我最初期望这样做，如果它是一个数组文字， [undefined，undefined，undefined] .map（fn）

What's going on here? Why do these allocated cells behave differently from the initialization? I was originally expecting this to act as it would if it was an array literal, [undefined, undefined, undefined].map(fn)

推荐答案

Array.map（） 不会为未定义其值的索引调用回调。

Array.map() does not invoke the callback for indexes whose values aren't defined.

来自MDN文档：

map为每个调用一次提供的回调函数
数组中的元素按顺序，并从结果中构造一个新数组。仅对已分配值的数组的索引调用callback
;
未调用未定义的索引，已删除
的索引或从未分配过的值。

map calls a provided callback function once for each element in an array, in order, and constructs a new array from the results. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes that are undefined, those which have been deleted or which have never been assigned values.

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