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.

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).

The first one is the default object constructor call. You can use it's parameters if you want.

var array = new Array(5); //initialize with default length 5

The second one gives you the ability to create not empty array:

var array = [1, 2, 3]; // this array will contain numbers 1, 2, 3.

There is no difference when you initialise array without any length. So var a = [] & var b = new Array() is same.

But if you initialise array with length like var b = new Array(1);, it will set array object's length to 1. So its equivalent to var b = []; b.length=1;.

This will be problematic whenever you do array_object.push, it add item after last element & increase length.

var b = new Array(1);
b.push("hello world");
console.log(b.length); // print 2

vs

var v = [];
a.push("hello world");
console.log(b.length); // print 1

There's more to this than meets the eye. Most other answers are correct BUT ALSO..

new Array(n)

• Allows engine to reallocates space for n elements
• Optimized for array creation
• Created array is marked sparse which has the least performant array operations, that's because each index access has to check bounds, see if value exists and walk the prototype chain
• If array is marked as sparse, there's no way back (at least in V8), it'll always be slower during its lifetime, even if you fill it up with content (packed array) 1ms or 2 hours later, doesn't matter

[1, 2, 3] || []

• Created array is marked packed (unless you use delete or [1,,3] syntax)
• Optimized for array operations (for .., forEach, map, etc)
• Engine needs to reallocate space as the array grows

This probably isn't the case for older browser versions/browsers.

I've incurred in a weird behaviour using [].

We have Model "classes" with fields initialised to some value. E.g.:

require([
  "dojo/_base/declare",
  "dijit/_WidgetBase",
], function(declare, parser, ready, _WidgetBase){

   declare("MyWidget", [_WidgetBase], {
     field1: [],
     field2: "",
     function1: function(),
     function2: function()
   });    
});

I found that when the fields are initialised with [] then it would be shared by all Model objects. Making changes to one affects all others.

This doesn't happen initialising them with new Array(). Same for the initialisation of Objects ({} vs new Object())

TBH I am not sure if its a problem with the framework we were using (Dojo)

Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.

Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.

Now let's see what are the difference

The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length. For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.

The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.

Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).

I hope that has explained enough about the beast new Array() :)

I found a difference while using promises. While using array of promises (say arr, initialised as arr=[]), got an error in Promise.all(arr). Whereas when declared as arr = Array(), did not get compilation issues. Hope this helps.