How to initialize an array's length in JavaScript?

Array(5) gives you an array with length 5 but no values, hence you can't iterate over it.

Array.apply(null, Array(5)).map(function () {}) gives you an array with length 5 and undefined as values, now it can be iterated over.

Array.apply(null, Array(5)).map(function (x, i) { return i; }) gives you an array with length 5 and values 0,1,2,3,4.

Array(5).forEach(alert) does nothing, Array.apply(null, Array(5)).forEach(alert) gives you 5 alerts

ES6 gives us Array.from so now you can also use Array.from(Array(5)).forEach(alert)

If you want to initialize with a certain value, these are good to knows... Array.from('abcde'), Array.from('x'.repeat(5)) or Array.from({length: 5}, (v, i) => i) // gives [0, 1, 2, 3, 4]

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Why do you want to initialize the length? Theoretically there is no need for this. It can even result in confusing behavior, because all tests that use the length to find out whether an array is empty or not will report that the array is not empty. Some tests show that setting the initial length of large arrays can be more efficient if the array is filled afterwards, but the performance gain (if any) seem to differ from browser to browser. jsLint does not like new Array() because the constructer is ambiguous. new Array(4); creates an empty array of length 4. But new Array('4'); creates an array containing the value '4'.

Regarding your comment: In JS you don't need to initialize the length of the array. It grows dynamically. You can just store the length in some variable, e.g.

var data = [];
var length = 5; // user defined length

for(var i = 0; i < length; i++) {
    data.push(createSomeObject());
}

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With ES2015 .fill() you can now simply do:

// `n` is the size you want to initialize your array
// `0` is what the array will be filled with (can be any other value)
Array(n).fill(0)

Which is a lot more concise than Array.apply(0, new Array(n)).map(i => value)

It is possible to drop the 0 in .fill() and run without arguments, which will fill the array with undefined. (However, this will fail in Typescript)

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[...Array(6)].map(x => 0);
// [0, 0, 0, 0, 0, 0]

OR

Array(6).fill(0);
// [0, 0, 0, 0, 0, 0]

Note: you can't loop empty slots i.e. Array(4).forEach(() => …)

OR

( typescript safe )

Array(6).fill(null).map((_, i) => i);
// [0, 1, 2, 3, 4, 5]

OR

Classic method using a function ( works in any browser )

function NewArray(size) {
    var x = [];
    for (var i = 0; i < size; ++i) {
        x[i] = i;
    }
    return x;
}

var a = NewArray(10);
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Creating nested arrays

When creating a 2D array with the fill intuitively should create new instances. But what actually going to happen is the same array will be stored as a reference.

var a = Array(3).fill([6]);
// [  [6], [6], [6]  ]

a[0].push(9);
// [  [6, 9], [6, 9], [6, 9]  ]

Solution

var a = [...Array(3)].map(x => []);

a[0].push(4, 2);
// [  [4, 2], [], []  ]

So a 3x2 Array will look something like this:

[...Array(3)].map(x => Array(2).fill(0));
// [  [0, 0], [0, 0], [0, 0]  ]

N-dimensional array

function NArray(...dimensions) {
    var index = 0;
    function NArrayRec(dims) {
        var first = dims[0], next = dims.slice().splice(1); 
        if(dims.length > 1) 
            return Array(dims[0]).fill(null).map((x, i) => NArrayRec(next ));
        return Array(dims[0]).fill(null).map((x, i) => (index++));
    }
    return NArrayRec(dimensions);
}

var arr = NArray(3, 2, 4);
// [   [  [ 0,  1,  2,  3 ] , [  4,  5,  6,  7]  ],
//     [  [ 8,  9,  10, 11] , [ 12, 13, 14, 15]  ],
//     [  [ 16, 17, 18, 19] , [ 20, 21, 22, 23]  ]   ]

Initialize a chessboard

var Chessboard = [...Array(8)].map((x, j) => {
    return Array(8).fill(null).map((y, i) => {
        return `${String.fromCharCode(65 + i)}${8 - j}`;
    });
});

// [ [A8, B8, C8, D8, E8, F8, G8, H8],
//   [A7, B7, C7, D7, E7, F7, G7, H7],
//   [A6, B6, C6, D6, E6, F6, G6, H6],
//   [A5, B5, C5, D5, E5, F5, G5, H5],
//   [A4, B4, C4, D4, E4, F4, G4, H4],
//   [A3, B3, C3, D3, E3, F3, G3, H3],
//   [A2, B2, C2, D2, E2, F2, G2, H2],
//   [A1, B1, C1, D1, E1, F1, G1, H1] ]

Math filled values

handy little method overload when working with math

function NewArray( size , method, linear )
{
    method = method || ( i => i ); 
    linear = linear || false;
    var x = [];
    for( var i = 0; i < size; ++i )
        x[ i ] = method( linear ? i / (size-1) : i );
    return x;
}

NewArray( 4 ); 
// [ 0, 1, 2, 3 ]

NewArray( 4, Math.sin ); 
// [ 0, 0.841, 0.909, 0.141 ]

NewArray( 4, Math.sin, true );
// [ 0, 0.327, 0.618, 0.841 ]

var pow2 = ( x ) => x * x;

NewArray( 4, pow2 ); 
// [ 0, 1, 4, 9 ]

NewArray( 4, pow2, true ); 
// [ 0, 0.111, 0.444, 1 ]

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The shortest:

let arr = [...Array(10)]; console.log(arr);

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ES6 introduces Array.from which lets you create an Array from any "array-like" or iterables objects:

Array.from({length: 10}, (x, i) => i);
// [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In this case {length: 10} represents the minimal definition of an "array-like" object: an empty object with just a length property defined.

Array.from allows for a second argument to map over the resulting array.

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Sparse arrays are here! 🥳 [2021]

In modern JS engines, sparse arrays are fully supported. You can use [] or new Array(len) in any way you like, even with random access. Dictionary mode seems to be a thing of the past.

In current Chrome (and I guess any V8 environment), Arrays can have a length of up to 2^32-1 and allocation is sparse (meaning empty chunks don't use up any memory):

https://i.stack.imgur.com/aEmtE.png

https://i.stack.imgur.com/db75k.png

However, there is a catch

On the one hand, for loops work as intended, however, Array's builtin higher order functions (such as map, filter, find, some etc.) ignore unassigned elements. They require fill (or some other method of population) first:

const a = new Array(10);
const b = new Array(10).fill(0);

a.forEach(x => console.log(x)); // does nothing
b.forEach(x => console.log(x)); // works as intended

Old Version

(I removed most of the old version.) The gist was that creating a large array using new Array(largeNumber) or random accessing an array in places that have not yet been allocated would tumble it into "dictionary mode". Meaning you are using an array with indexes, but under the hood it would use a dictionary to store the values, thus messing with performance, and also with iteration behavior. Luckily that is a thing of the past.

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This will initialize the length property to 4:

var x = [,,,,];

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I'm surprised there hasn't been a functional solution suggested that allows you to set the length in one line. The following is based on UnderscoreJS:

var test = _.map(_.range(4), function () { return undefined; });
console.log(test.length);

For reasons mentioned above, I'd avoid doing this unless I wanted to initialize the array to a specific value. It's interesting to note there are other libraries that implement range including Lo-dash and Lazy, which may have different performance characteristics.

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Here is another solution

var arr = Array.apply( null, { length: 4 } );
arr;  // [undefined, undefined, undefined, undefined] (in Chrome)
arr.length; // 4

The first argument of apply() is a this object binding, which we don't care about here, so we set it to null.

Array.apply(..) is calling the Array(..) function and spreading out the { length: 3 } object value as its arguments.

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Please people don't give up your old habits just yet. There is a large difference in speed between allocating memory once then working with the entries in that array (as of old), and allocating it many times as an array grows (which is inevitably what the system does under the hood with other suggested methods).

None of this matters of course, until you want to do something cool with larger arrays. Then it does.

Seeing as there still seems to be no option in JS at the moment to set the initial capacity of an array, I use the following...

var newArrayWithSize = function(size) {
  this.standard = this.standard||[];
  for (var add = size-this.standard.length; add>0; add--) {
   this.standard.push(undefined);// or whatever
  }
  return this.standard.slice(0,size);
}

There are tradeoffs involved:

This method takes as long as the others for the first call to the function, but very little time for later calls (unless asking for a bigger array).

The standard array does permanently reserve as much space as the largest array you have asked for.

But if it fits with what you're doing there can be a payoff. Informal timing puts

for (var n=10000;n>0;n--) {var b = newArrayWithSize(10000);b[0]=0;}

at pretty speedy (about 50ms for the 10000 given that with n=1000000 it took about 5 seconds), and

for (var n=10000;n>0;n--) {
  var b = [];for (var add=10000;add>0;add--) {
    b.push(undefined);
  }
}

at well over a minute (about 90 sec for the 10000 on the same chrome console, or about 2000 times slower). That won't just be the allocation, but also the 10000 pushes, for loop, etc..

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(this was probably better as a comment, but got too long)

So, after reading this I was curious if pre-allocating was actually faster, because in theory it should be. However, this blog gave some tips advising against it http://www.html5rocks.com/en/tutorials/speed/v8/.

So still being unsure, I put it to the test. And as it turns out it seems to in fact be slower.

var time = Date.now();
var temp = [];
for(var i=0;i<100000;i++){
    temp[i]=i;
}
console.log(Date.now()-time);


var time = Date.now();
var temp2 = new Array(100000);
for(var i=0;i<100000;i++){
    temp2[i] = i;
}
console.log(Date.now()-time); 

This code yields the following after a few casual runs:

$ node main.js 
9
16
$ node main.js 
8
14
$ node main.js 
7
20
$ node main.js 
9
14
$ node main.js 
9
19

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var arr=[];
arr[5]=0;
alert("length="+arr.length); // gives 6

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Assuming that Array's length is constant. In Javascript, This is what we do:

const intialArray = new Array(specify the value);

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The array constructor has an ambiguous syntax, and JSLint just hurts your feelings after all.

Also, your example code is broken, the second var statement will raise a SyntaxError. You're setting the property length of the array test, so there's no need for another var.

As far as your options go, array.length is the only "clean" one. Question is, why do you need to set the size in the first place? Try to refactor your code to get rid of that dependency.

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In addition to the answers of others, another clever way is to use Float32Array to create an array and iterate on it.

For this purpose, create an instance from Float32Array with your desired length like this:

new Float32Array(5)

This code returns an array-like that you can convert it to an array with Array.from():

Array.from(new Float32Array(5)) // [0, 0, 0, 0, 0]

You can also use fill() to change the value of items:

Array.from(new Float32Array(5).fill(2)) // [2, 2, 2, 2, 2]

And of course you can iterate on it:

Array.from(new Float32Array(5)).map(item => /* ... */ )

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In most answers it is recommended to fill the array because otherwise "you can't iterate over it", but this is not true. You can iterate an empty array, just not with forEach. While loops, for of loops and for i loops work fine.

const count = Array(5);

Does not work.

console.log('---for each loop:---');
count.forEach((empty, index) => {
    console.log(`counting ${index}`);
});

These work:

console.log('---for of loop:---');
for (let [index, empty] of count.entries()) {
  console.log(`counting for of loop ${index}`);
}

console.log('---for i loop:---');
for (let i = 0, il = count.length; i < il; ++i) {
  console.log(`counting for i loop ${i}`);
}

console.log('---while loop:---');
let index = 0;
while (index < count.length) { 
  console.log(`counting while loop ${index}`); 
  index++; 
}

Check this fiddle with the above examples.

Also angulars *ngFor works fine with an empty array:

<li *ngFor="let empty of count; let i = index" [ngClass]="
  <span>Counting with *ngFor {{i}}</span>
</li>

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You can set the array length by using array.length = youValue

So it would be

var myArray = [];
myArray.length = yourValue;

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The reason you shouldn't use new Array is demonstrated by this code:

var Array = function () {};

var x = new Array(4);

alert(x.length);  // undefined...

Some other code could mess with the Array variable. I know it's a bit far fetched that anyone would write such code, but still...

Also, as Felix King said, the interface is a little inconsistent, and could lead to some very difficult-to-track-down bugs.

If you wanted an array with length = x, filled with undefined (as new Array(x) would do), you could do this:

var x = 4;
var myArray = [];
myArray[x - 1] = undefined;

alert(myArray.length); // 4

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