Array.prototype.fill() With Object Passes Reference And Not New Instance

Answer :

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); 

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.

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