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

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:

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

This works well if the value of y is anything other than an object. Meaning that if y is an object, the following is true:

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

Is there any way to state that a new object should be created for each element while using the fill-function? Or should I just convert it to a loop?

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] = [];
    }
    // or it's one line alternative
    for (var i = 0, a = []; i < ARRAY_SIZE; 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):

   // Standard multi-line way
   a = new Array(ARRAY_SIZE);
   for (var i = 0; i < ARRAY_SIZE; i++) {
       a[i] = [];
   }
  
   // One line syntax
   for (var i = 0, a = []; i < ARRAY_SIZE; 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())

Shortest Possable:

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

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.

You could also solve this by the following workaround.

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

I wrote a blog post about this: http://www.samhenderson.xyz/posts/12

But the TLDR is that if you want to avoid chaining multiple function e.g. fill, map. And want to avoid writing a loop, then you can use:

const array = Array.from({ length: 2 },()=>({}))

For an array of arrays:

const array = Array.from({ length: 2 },()=>([]))