Array.from({length:5}，(v，i)=> i)`如何工作? [英] How does `Array.from({length: 5}, (v, i) => i)` work?

问题描述

我可能在这里遗漏了一些明显的东西，但是有人可以逐步解释为什么Array.from({length: 5}, (v, i) => i)返回[0, 1, 2, 3, 4]的原因吗?

I may be missing something obvious here but could someone breakdown step by step why Array.from({length: 5}, (v, i) => i) returns [0, 1, 2, 3, 4]?

https://developer.mozilla.org/zh_CN/docs/Web/JavaScript/Reference/Global_Objects/Array/from

我不明白为什么会这样

推荐答案

当Javascript检查是否可以调用方法时，它会使用鸭型.这意味着，当您想从某个对象(其类型为bar)调用方法foo时，它不会检查该对象是否真的为bar，但会检查它是否具有方法.

When Javascript checks if a method can be called, it uses duck-typing. That means when you want to call a method foo from some object, which is supposed to be of type bar, then it doesn't check if this object is really bar but it checks if it has method foo.

因此在JS中，可以执行以下操作:

So in JS, it's possible to do the following:

let fakeArray = {length:5};
fakeArray.length //5
let realArray = [1,2,3,4,5];
realArray.length //5

第一个类似于 fake javascript数组(具有属性length).当Array.from获得属性length的值(在这种情况下为5)时，它将创建一个长度为5的实数组.

First one is like fake javascript array (which has property length). When Array.from gets a value of property length (5 in this case), then it creates a real array with length 5.

这种 fakeArray 对象通常称为 arrayLike .

第二部分只是一个箭头函数，该函数使用索引值(第二个参数)填充数组.

The second part is just an arrow function which populates an array with values of indices (second argument).

此技术对于模拟某些对象进行测试非常有用.例如:

This technique is very useful for mocking some object for test. For example:

let ourFileReader = {}
ourFileReader.result = "someResult"
//ourFileReader will mock real FileReader

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