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

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问题描述

我可能在这里遗漏了一些明显的东西,但有人可以逐步分解为什么 Array.from({length: 5}, (v, i) => i) 返回 [0, 1, 2, 3, 4]?

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/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from

我没有详细理解为什么会这样

I didn't understand in detail why this works

推荐答案

当 Javascript 检查一个方法是否可以被调用时,它使用 鸭打字.这意味着当你想从某个对象调用方法 foo 时,它应该是 bar 类型,然后它不会检查这个对象是否真的是 bar 但它检查它是否有方法 foo.

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 获取属性 length 的值(在本例中为 5)时,它会创建一个长度为 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对象通常被称为arrayLike.

第二部分只是一个箭头函数,它用索引值(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).

这种技术对于模拟一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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