Javascript Array constructor issue

Question

I'm wondering about why isn't advisable to use array declaration by:

 var arr = new Array()

as I thought declaring by [] was supposed to be protected in case of overwrite Array but...

Array = 1;
var arr = [] // boom  
TypeError: Cannot read property 'slice' of undefined

Personally, I prefer using var arr = [], but now I don't know what is the advantage of using [] instead of Array except that write faster.

Mike Samuel · Accepted Answer · 2012-01-07 15:55:29Z

4

as I thought declaring by [] was supposed to be protected in case of overwrite Array but...

It is under EcmaScript 5 but not under EcmaScript 3, so it won't be on older browsers.

The relevant portion of the spec is http://es5.github.com/#x11.1.4

Let array be the result of creating a new object as if by the expression new Array() where Array is the standard built-in constructor with that name.

The bolded text was added in EcmaScript 5 but was not present in EcmaScript 3.

answered Jan 7, 2012 at 15:55

Mike Samuel

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Comments

Rob W · Accepted Answer · 2012-01-07 15:48:59Z

4

Both methods are interchangeable (regarding functionailty), except for one case:

These are not equivalent:

var array = new Array(3);  // <-- Creates an array with length 3
var array = [3];

//If you wanted to use the `Array` to create an array with one element, use:
var array = new Array();
array[0] = 3;

[] is not only shorter, but also consistent. The Array constructor is only useful for creating arrays with an initial length.

answered Jan 7, 2012 at 15:48

Rob W

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1 Comment

Raynos Over a year ago

Note [] is favoured because 1) it minimizes better and 2) new Array() has a functional call overhead where as [] can be optimized better internally in javascript engines.

Community · Accepted Answer · 2017-05-23 12:23:50Z

2

There's already lots of similar posts on SO.

See this:

Javascript array best practice to use [] instead of new array?

And this:

Use of JavaScript new Array(n) Declaration

edited May 23, 2017 at 12:23

CommunityBot

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answered Jan 7, 2012 at 15:43

Martin Dandanell

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2 Comments

Rob W Over a year ago

Instead of posting this as an answer, you can flag the answer for attention, and say "Duplicate of: <other question(s) here>". When you get the privilege to add comments, this would also be suitable for a comment.

Martin Dandanell Over a year ago

Will do - Thanks for the heads up!

Osman Turan · Accepted Answer · 2012-01-07 15:42:43Z

0

IMO, it doesn't really matter you use new Array() or =[]. Because, most of today's browsers' Javascript engines are smart enough to take advantages of small implementation differences. I frequently use =[] just because of shave off some extra bytes from the javascript source.

answered Jan 7, 2012 at 15:42

Osman Turan

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Comments

TJR · Accepted Answer · 2012-01-08 00:01:02Z

0

I ran a test. It appears that if you override window.Array with anything except another function it won't work.

answered Jan 8, 2012 at 0:01

TJR

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Comments

user1120193 · Accepted Answer · 2012-01-07 15:42:28Z

-5

[] it doesn't mean anything in fact Javascript doesn't explicitly mention anything like that. so you have to follow the variable declaration rules as its standard mentions.

answered Jan 7, 2012 at 15:42

user1120193

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5 Comments

user1120193 Over a year ago

don't waste your time with this I suggest you move up and learn rest of concepts of Javascript .

Dave Newton Over a year ago

It still doesn't address the question, though; maybe expand on what you're trying to say a little.

user1120193 Over a year ago

I just show the direction and you have to travel by yourself.

user1120193 Over a year ago

I am not here for those tiny pixelated badges and reputations. perhaps people like you can have some fun thumbing down on me but I don't care.

Rob W Over a year ago

@Gryphes I did not vote down, because -3 is enough to fade an answer. I encourage wrong answers to be deleted, so that SO remains clean.

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