[javascript] 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 = [];

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

I can explain in a more specific way starting with this example that's based on Fredrik's good one.

var test1 = [];
test1.push("value");
test1.push("value2");

var test2 = new Array();
test2.push("value");
test2.push("value2");

alert(test1);
alert(test2);
alert(test1 == test2);
alert(test1.value == test2.value);

I just added another value to the arrays, and made four alerts: The first and second are to give us the value stored in each array, to be sure about the values. They will return the same! Now try the third one, it returns false, that's because

JS treats test1 as a VARIABLE with a data type of array, and it treats test2 as an OBJECT with the functionality of an array, and there are few slight differences here.

The first difference is when we call test1 it calls a variable without thinking, it just returns the values that are stored in this variable disregarding its data type! But, when we call test2 it calls the Array() function and then it stores our "Pushed" values in its "Value" property, and the same happens when we alert test2, it returns the "Value" property of the array object.

So when we check if test1 equals test2 of course they will never return true, one is a function and the other is a variable (with a type of array), even if they have the same value!

To be sure about that, try the 4th alert, with the .value added to it; it will return true. In this case we tell JS "Disregarding the type of the container, whether was it function or variable, please compare the values that are stored in each container and tell us what you've seen!" that's exactly what happens.

I hope I said the idea behind that clearly, and sorry for my bad English.

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...

Using the Array constructor makes a new array of the desired length and populates each of the indices with undefined, the assigned an array to a variable one creates the indices that you give it info for.

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]

There is no big difference, they basically do the same thing but doing them in different ways, but read on, look at this statement at W3C:

var cars = ["Saab", "Volvo","BMW"];

and

var cars = new Array("Saab", "Volvo", "BMW");

The two examples above do exactly the same. There is no need to use new Array().
For simplicity, readability and execution speed, use the first one (the array literal method).

But at the same time, creating new array using new Array syntax considered as a bad practice:

Avoid new Array()

There is no need to use the JavaScript's built-in array constructor new Array().
Use [] instead.
These two different statements both create a new empty array named points:

var points = new Array();         // Bad
var points = [];                  // Good 

These two different statements both create a new array containing 6 numbers:

var points = new Array(40, 100, 1, 5, 25, 10); // Bad    
var points = [40, 100, 1, 5, 25, 10];          // Good

The new keyword only complicates the code. It can also produce some unexpected results:

var points = new Array(40, 100);  // Creates an array with two elements (40 and 100)

What if I remove one of the elements?

var points = new Array(40);       // Creates an array with 40 undefined elements !!!!!

So basically not considered as the best practice, also there is one minor difference there, you can pass length to new Array(length) like this, which also not a recommended way.

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.

The first one is the default object constructor call.mostly used for dynamic values.

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

the second array is used when creating static values

var array = [red, green, blue, yellow, white]; // this array will contain values.

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).

I've found one difference between the two constructions that bit me pretty hard.

Let's say I have:

function MyClass(){
  this.property1=[];
  this.property2=new Array();
};
var MyObject1=new MyClass();
var MyObject2=new MyClass();

In real life, if I do this:

MyObject1.property1.push('a');
MyObject1.property2.push('b');
MyObject2.property1.push('c');
MyObject2.property2.push('d');

What I end up with is this:

MyObject1.property1=['a','c']
MyObject1.property2=['b']
MyObject2.property1=['a','c']
MyObject2.property2=['d']

I don't know what the language specification says is supposed to happen, but if I want my two objects to have unique property arrays in my objects, I have to use new Array().

As I know the diference u can find the slice(or the other funcitons of Array) like code1.and code2 show u Array and his instances:

code1:

[].slice; // find slice here
var arr = new Array();
arr.slice // find slice here
Array.prototype.slice // find slice here

code2:

[].__proto__ == Array.prototype; // true
var arr = new Array();
arr.__proto__ == Array.prototype; // true

conclusion:

as u can see [] and new Array() create a new instance of Array.And they all get the prototype functions from Array.prototype

They are just different instance of Array.so this explain why [] != []

:)

The difference of using

var arr = new Array(size);

Or

arr = [];
arr.length = size;

As been discussed enough in this question.

I would like to add the speed issue - the current fastest way, on google chrome is the second one.

But pay attention, these things tend to change a lot with updates. Also the run time will differ between different browsers.

For example - the 2nd option that i mentioned, runs at 2 million [ops/second] on chrome, but if you'd try it on mozilla dev. you'd get a surprisingly higher rate of 23 million.

Anyway, I'd suggest you check it out, every once in a while, on different browsers (and machines), using site as such

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() :)

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.

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.

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)

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.