"Array.prototype.fill() uses objects to pass references instead of creating new instances"

Question

I'm playing around a bit and trying to instantiate a new array of length x where all elements of the array are initialized to the value y: vararr=newArray(x).fill(y); If the value of y is not an object then this method Can work very well. That is to say, if y is an object, then the following conditions hold: vararr=newArray(2).fill({});arr[0]===arr[1];//The result is true;arr[0] .test='string';arr[1].test

P粉878510551 · Answer

The accepted answer is great and works 90% of the time.

However, if you are making a high-performance JS application and are using large/huge arrays, Array.map(..) will create a lot of load in terms of memory and processor usage, Because it creates a copy of the array.

I recommend using the classic for loop:

a = new Array(ARRAY_SIZE);
    for (var i = 0; i < ARRAY_SIZE; i++) {
        a[i] = [];
    }
    // 或者使用一行代码的替代方案
    for (var i = 0, a = []; i < ARRAY_SIZE; a[i++] = []);

I tested six alternatives and got the following results:

Array.map(), as above (11 times slower than the original! ):
```
a = new Array(ARRAY_SIZE).fill().map(u => { return []; });
```

for loop, best choice (fastest):

// 标准的多行方式
 a = new Array(ARRAY_SIZE);
 for (var i = 0; i < ARRAY_SIZE; i++) {
     a[i] = [];
 }

 // 一行语法
 for (var i = 0, a = []; i < ARRAY_SIZE; a[i++] = []);

forEach (slower 6 times ):

a = new Array(ARRAY_SIZE).fill();
 a.forEach((val, i) => {
     a[i] = [];
 })

[Updated on 2020-08-27] Ilias Karim proposed another method

Array.from (30 times slower! ) - Definitely worse in terms of performance, despite having the best syntax :(
```
a = Array.from({ length: ARRAY_SIZE }, () => []);
```
[..Array(..)] (5 times slower!)
```
a = [...Array(ARRAY_SIZE)].map(_=>([]))
```
Array.push(..), ranked second in terms of performance (2x slower! )
```
let a = [], total = ARRAY_SIZE;
 while(total--) a.push([]);
```

PS: I tested on this fiddle.

P粉461599845 · Answer

You can first fill the array with any value (e.g. undefined) and then you can use map:

var arr = new Array(2).fill().map(u => ({}));

var arr = new Array(2).fill().map(Object);