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

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

What’s the real difference between declaring an array like this:
var myArray = new Array();

and
var myArray = [];

Solutions/Answers:

Solution 1:

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.

Solution 2:

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…

Solution 3:

There is a huge difference that no one mentioned.

You might think the new Array(2) is equivalent to [undefined, undefined] before because we have

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

BUT IT’S NOT!

Let’s try map():

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

See? It’s different! But why is that?

According to ES6 Spec 22.1.1.2, Array(len) only creates a new array with length set to len and nothing more. Thus there is no real element inside the new array.

While 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

That’s why you can not expect any iteration functions work as usual to array created from new Array(len).

BTW, Safari and Firefox have a much better expression to this:

// 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 Chrome to ask them to fix this confusing log:
https://bugs.chromium.org/p/chromium/issues/detail?id=732021

UPDATE: It’s already fixed. Chrome now log as

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

Solution 4:

Oddly enough, new Array(size) is almost 2x faster than [] in Chrome, and about the same in FF and IE (measured by creating and filling an array). It only matters if you know the approximate size of the array. If you add more items than the length you’ve given, the performance boost is lost.

More accurately: Array( is a fast constant time operation that allocates no memory, wheras [] is a linear time operation that sets type and value.

Solution 5:

For more information, the following page describes why you never need to use new Array()

You never need to use new Object() in
JavaScript. Use the object literal {}
instead. Similarly, don’t use new Array(),
use the array literal []
instead. Arrays in JavaScript work
nothing like the arrays in Java, and
use of the Java-like syntax will
confuse you.

Do not use new Number, new String, or
new Boolean. These forms produce
unnecessary object wrappers. Just use
simple literals instead.

Also check out the comments – the new Array(length) form does not serve any useful purpose (at least in today’s implementations of JavaScript).

Solution 6:

In order to better understand [] and new Array():

> []
  []
> new Array()
  []
> [] == []
  false
> [] === []
  false
> new Array() == new Array()
  false
> new Array() === new Array()
  false
> typeof ([])
  "object"
> typeof (new Array())
  "object"
> [] === new Array()
  false
> [] == new Array()
  false

The above result is from Google Chrome console on Windows 7.