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

Question

I was playing around a bit, trying to instantiate a new array of length x, where all elements of the array were initialized to the value y:< /p>

var arr = new Array(x).fill(y);

This method works well if the value of y is not an object. That is, if y is an object, then the following conditions are true:

var arr = new Array(2).fill({});
arr[0] === arr[1]; //The result is true;
arr[0].test = 'string';
arr[1].test === 'string'; //The result is also true;

Is there a way to declare that a new object should be created for each element when using the fill function? Or should I convert it to a loop?

Answer 1

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.

Answer 2

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);