为什么我们在`map()`函数中使用`Number.prototype.valueOf` [英] Why do we use `Number.prototype.valueOf` inside of a `map()` function

问题描述

以下代码:

let resultsArray = Array.apply(null, Array(10)).map(Number.prototype.valueOf,0);

创建以下数组

[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

为什么map()只需要将数字0推入此数组的每个位置，都需要Number.prototype.valueOf.有没有其他方法(更有效)来达到这个结果，或者这是最好的方法?

Why does map() need Number.prototype.valueOf just to push the number 0 in to each position of this array. Is there a different (more efficient) way to achieve this result, or is this the best way?

推荐答案

如果您阅读

If you read the map documentation you can read this:

map()方法创建一个新数组，其结果是对该数组中的每个元素调用提供的函数.

The map() method creates a new array with the results of calling a provided function on every element in this array.

因此，您需要在第一个参数中使用函数 Number.prototype.valueOf ，在第二个可选参数中使用Number对象来填充新数组. Number对象用作第一个参数的this.

So you need to use the function Number.prototype.valueOf in first parameter and a Number object in second optional parameter to fill the new array. The Number object is used as the this of the first parameter.

您也可以为相同的结果编写以下代码:

You can also write this for the same result:

let resultsArray = Array.apply(null, Array(10)).map(function(){return 0;});

但是，如果您只是想填充一个带有值的数组，我认为您可以使用

But if you juste want to fill an array with a value, I think you may use the Array.prototype.fill method.

fill()方法使用静态值填充数组的所有元素，从开始索引到结束索引.

The fill() method fills all the elements of an array from a start index to an end index with a static value.

let resultsArray = (new Array(10)).fill(0);

---

性能测试:

---

Performance test:

var start, stop, array;
var iteration = 100000;


// Map
start = Date.now();
array = Array.apply(null, Array(iteration)).map(Number.prototype.valueOf,0);
stop = Date.now();
console.log("Map executed in "+(stop-start)+"ms");

// Map simple array
start = Date.now();
array = (new Array(iteration)).map(Number.prototype.valueOf,0);
stop = Date.now();
console.log("Map simple array executed in "+(stop-start)+"ms");

// Map simple function
start = Date.now();
array = (new Array(iteration)).map(function(){return 0;});
stop = Date.now();
console.log("Map simple function executed in "+(stop-start)+"ms");

// Array.from - ES6 from @Zohaib ijaz
start = Date.now();
array = Array.from(new Array(iteration), () => 0)
stop = Date.now();
console.log("Array.from - ES6 from @Zohaib ijaz executed in "+(stop-start)+"ms");

// Array.from - Non-ES6 from @Zohaib ijaz
start = Date.now();
array = Array.from(new Array(iteration), function(){return 0;})
stop = Date.now();
console.log("Array.from - Non-ES6 from @Zohaib ijaz executed in "+(stop-start)+"ms");

// repeat-split-map by @nicael
start = Date.now();
array = '0'.repeat(iteration).split('').map(Number);
stop = Date.now();
console.log("repeat-split-map by @nicael executed in "+(stop-start)+"ms");

// Fill
start = Date.now();
array = (new Array(iteration)).fill(0);
stop = Date.now();
console.log("Fill executed in "+(stop-start)+"ms");

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