What’s the difference between "Array()" and "[]" while declaring a JavaScript array?

There is a difference, but there is no difference in that example.

Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:

x = new Array(5);
alert(x.length); // 5

To illustrate the different ways to create an array:

var a = [],            // these are the same
    b = new Array(),   // a and b are arrays with length 0

    c = ['foo', 'bar'],           // these are the same
    d = new Array('foo', 'bar'),  // c and d are arrays with 2 strings

    // these are different:
    e = [3]             // e.length == 1, e[0] == 3
    f = new Array(3),   // f.length == 3, f[0] == undefined

;

Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.

As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.


The difference between creating an array with the implicit array and the array constructor is subtle but important.

When you create an array using

var a = [];

You're telling the interpreter to create a new runtime array. No extra processing necessary at all. Done.

If you use:

var a = new Array();

You're telling the interpreter, I want to call the constructor "Array" and generate an object. It then looks up through your execution context to find the constructor to call, and calls it, creating your array.

You may think "Well, this doesn't matter at all. They're the same!". Unfortunately you can't guarantee that.

Take the following example:

function Array() {
    this.is = 'SPARTA';
}

var a = new Array();
var b = [];

alert(a.is);  // => 'SPARTA'
alert(b.is);  // => undefined
a.push('Woa'); // => TypeError: a.push is not a function
b.push('Woa'); // => 1 (OK)

In the above example, the first call will alert 'SPARTA' as you'd expect. The second will not. You will end up seeing undefined. You'll also note that b contains all of the native Array object functions such as push, where the other does not.

While you may expect this to happen, it just illustrates the fact that [] is not the same as new Array().

It's probably best to just use [] if you know you just want an array. I also do not suggest going around and redefining Array...


There is an important difference that no answer has mentioned yet.

From this:

new Array(2).length           // 2
new Array(2)[0] === undefined // true
new Array(2)[1] === undefined // true

You might think the new Array(2) is equivalent to [undefined, undefined], but it's NOT!

Let's try with map():

[undefined, undefined].map(e => 1)  // [1, 1]
new Array(2).map(e => 1)            // "(2) [undefined × 2]" in Chrome

See? The semantics are totally different! So why is that?

According to ES6 Spec 22.1.1.2, the job of Array(len) is just creating a new array whose property length is set to the argument len and that's it, meaning there isn't any real element inside this newly created array.

Function map(), according to spec 22.1.3.15 would firstly check HasProperty then call the callback, but it turns out that:

new Array(2).hasOwnProperty(0) // false
[undefined, undefined].hasOwnProperty(0) // true

And that's why you can not expect any iterating functions working as usual on arrays created from new Array(len).

BTW, Safari and Firefox have a much better "printing" to this situation:

// Safari
new Array(2)             // [](2)
new Array(2).map(e => 1) // [](2) 
[undefined, undefined]   // [undefined, undefined] (2) 

// Firefox
new Array(2)             // Array [ <2 empty slots> ]
new Array(2).map(e => 1) // Array [ <2 empty slots> ]
[undefined, undefined]   // Array [ undefined, undefined ]

I have already submitted an issue to Chromium and ask them to fix this confusing printing: https://bugs.chromium.org/p/chromium/issues/detail?id=732021

UPDATE: It's already fixed. Chrome now printed as:

new Array(2)             // (2) [empty × 2]

Tags:

Javascript

Arrays

Declaration

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