Javascript - Initialize array with nulls

Question

In Javascript, why is

var myArray = new Array(3);

different from:

var otherArray = [null, null, null];

?

Obs: (myArray == otherArray) returns false.

And also, how can I get a variable like otherArray, which is an array full of 'nulls`, but with whatever size i'd like?

Obs

[undefined, undefined, undefined] 

is also not equal to myArray.

Answer 1

With EcmaScript 6 (ES2105), creating an array containing five nulls is as easy as this:

const arr = new Array(5).fill(null);

MDN Reference

Answer 2

This var myArray = new Array(3); will create an empty array. Hence, for this reason, myArray and otherArray are different arrays. Furthermore, even if they had the same values, three undefined values, the arrays wouldn't be the same. An array is an object and the variable myArray holds a reference to that object. Two objects with the same values aren't the same.

For instance,

var a = new Object();
var b = new Object();
console.log(a===b); // outputs false.

In addition to this:

var customerA = { name: "firstName" };
var customerB = { name: "firstName" };
console.log(customerA===customerB); // outputs false.

Update

Furthermore, in the case of var myArray = new Array(3) even the indices aren't initialized, as correctly Paul pointed out in his comment.

If you try this:

var array = [1,2,3];
console.log(Object.keys(array));

you will get as an output:

["1","2","3"];

While if you try this:

var array = new Array(3);
console.log(Object.keys(array));

you will get as output:

[]

Answer 3

The first point to note is that if you want to compare two Arrays or any other Object, you either have to loop over them or serialize them as comparing references will always give false

---

How can I get a variable like otherArray, which is an array full of 'nulls', but with whatever size I'd like?

Here is an alternative method for creating Arrays with a default value for its items and all indices initialised:

function createArray(len, itm) {
    var arr1 = [itm],
        arr2 = [];
    while (len > 0) {
        if (len & 1) arr2 = arr2.concat(arr1);
        arr1 = arr1.concat(arr1);
        len >>>= 1;
    }
    return arr2;
}

Now,

createArray(9, null);
// [null, null, null, null, null, null, null, null, null]

Answer 4

I've did some research and it turned out that the Array(length).fill(null) it not the best solution in terms of performance.

The best performance showed:

const nullArr = Array(length)
for (let i = 0; i < length; i++) {
  nullArr[i] = null
}

Just look at this:

const Benchmark = require('benchmark')
const suite = new Benchmark.Suite

const length = 10000

suite
  .add('Array#fill', function () {
    Array(length).fill(null)
  })
  .add('Array#for', function () {
    const nullArr = Array(length)
    for (let i = 0; i < length; i++) {
      nullArr[i] = null
    }
  })

  .on('cycle', function (event) {
    console.log(String(event.target))
  })
  .on('complete', function () {
    console.log('Fastest is ' + this.filter('fastest').map('name'))
  })

  .run({ async: true })

It shows the following results:

Array#fill x 44,545 ops/sec ±0.43% (91 runs sampled)
Array#for x 78,789 ops/sec ±0.35% (94 runs sampled)
Fastest is Array#for

Answer 5

You can also try [...new Array(76)] to generate 76 undefineds.

console.log(
  [...new Array(76)]
)  

Or to fill with null.

console.log(
  [...new Array(76).fill(null)]
)