Array.prototype.fill() with object passes reference and not new instance

Question

I was toying a bit and was trying to instantiate a new array of length x, where all elements of that array were initialized to a value y:

Answer 1

Ilias Karim's answer is most excellent. I just did the following:

a = Array.from({length:l}, () => new Array(c).fill(prefix));

to create a pre-filled 2D array of the specified size, l by c, filled with prefix. Now my code can fill in the slots in the 2D matrix that need non-prefix values.

Answer 2

Shortest Possable:

let node =  [...Array(2)].map(_=>({}))
console.log(node)

Answer 3

You could also solve this by the following workaround.

var arr = new Array(2).fill({});
arr = JSON.parse(JSON.stringify(arr));

Answer 4

One performant solution: Array.from({ length: 5 }, () => new Object())

Answer 5

The accepted answer is good and would work in 90% of cases.

But if you are making high-performance JS application, and if you work with big/huge arrays, Array.map(..) creates big overload in both - memory and processor use, as it creates a copy of an array.

I recommend to use the classic for loop:

    a = new Array(ARRAY_SIZE);
    for (var i = 0; i < ARRAY_SIZE; i++) {
        a[i] = [];
    }

I tested six alternatives and got this:

• Array.map(), as proposed above (11x times!!! slower):

   a = new Array(ARRAY_SIZE).fill().map(u => { return []; });
  

• for loop, the best one (fastest):

   a = new Array(ARRAY_SIZE);
   for (var i = 0; i < ARRAY_SIZE; i++) {
       a[i] = [];
   }
  

• forEach (6x time slower):

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

[UPDATE 2020-08-27] One more way proposed by Ilias Karim below

• Array.from (30x times!!! slower) - apparently worse in terms of performance, despite the nicest syntax :(

   a = Array.from({ length: ARRAY_SIZE }, () => []);
  

• [..Array(..)] (5x times!!! slower)

   a = [...Array(ARRAY_SIZE)].map(_=>([]))
  

• Array.push(..), second place in terms of performance (2x times!!! slower)

   let a = [], total = ARRAY_SIZE;
   while(total--) a.push([]);
  

PS. I used this fiddle for tests.

Answer 6

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

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

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