What is the way of declaring an array in JavaScript?

Question

I'm just learning JavaScript and it seems like there are a number of ways to declare arrays.

var myArray = new Array()
var myArray = new Array(3)
var myArray = ["apples", "bananas", "oranges"]
var myArray = [3]

What are their difference, and what are the preferred ways?

According to this website the following two lines are very different:

var badArray = new Array(10); // creates an empty Array that's sized for 10 elements
var goodArray= [10];          // creates an Array with 10 as the first element

As you can see these two lines do two very different things. If you had wanted to add more than one item then badArray would be initialized correctly since Javascript would then be smart enough to know that you were initializing the array instead of stating how many elements you wanted to add.

Is what the authors trying to say is Array(10) creates an array with precisely 10 elements and [10] creates an array of undefined size with the 0th element being 10? Or what does this mean?

#3 is the most common. You should read Crockford's "Javascript: The Good Parts" for a really solid foundation. — Mike Robinson
– Mike Robinson, Commented Jul 9, 2012 at 21:02

gen_Eric · Accepted Answer · 2012-07-09 21:06:06Z

38

In your first example, you are making a blank array, same as doing var x = []. The 2nd example makes an array of size 3 (with all elements undefined). The 3rd and 4th examples are the same, they both make arrays with those elements.

Be careful when using new Array().

var x = new Array(10); // array of size 10, all elements undefined
var y = new Array(10, 5); // array of size 2: [10, 5]

The preferred way is using the [] syntax.

var x = []; // array of size 0
var y = [10] // array of size 1: [1]

var z = []; // array of size 0
z[2] = 12;  // z is now size 3: [undefined, undefined, 12]

answered Jul 9, 2012 at 21:06

gen_Eric

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user1479055 · Accepted Answer · 2012-07-09 21:02:00Z

The preferred way is to always use the literal syntax with square brackets; its behaviour is predictable for any number of items, unlike Array's. What's more, Array is not a keyword, and although it is not a realistic situation, someone could easily overwrite it:

function Array() { return []; }

alert(Array(1, 2, 3)); // An empty alert box

However, the larger issue is that of consistency. Someone refactoring code could come across this function:

function fetchValue(n) {
    var arr = new Array(1, 2, 3);

    return arr[n];
}

As it turns out, only fetchValue(0) is ever needed, so the programmer drops the other elements and breaks the code, because it now returns undefined:

var arr = new Array(1);

Zack · Accepted Answer · 2018-04-21 18:28:01Z

13

To declare it:

var myArr = ["apples", "oranges", "bananas"];

To use it:

document.write("In my shopping basket I have " + myArr[0] + ", " + myArr[1] + ", and " + myArr[2]);

answered Apr 21, 2018 at 18:28

Zack

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1 Comment

P Ackerman Over a year ago

This doesn't really answer the question as specified at the end of the entry.

user664833 · Accepted Answer · 2016-09-19 04:04:59Z

8

There are a number of ways to create arrays.

The traditional way of declaring and initializing an array looks like this:

var a = new Array(5); // declare an array "a", of size 5
a = [0, 0, 0, 0, 0];  // initialize each of the array's elements to 0

Or...

// declare and initialize an array in a single statement
var a = new Array(0, 0, 0, 0, 0);

edited Sep 19, 2016 at 4:04

user664833

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answered Jul 15, 2013 at 21:46

Deepak

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3 Comments

hgoebl Over a year ago

IMHO this is exactly the way of how not to do it. Look at other answers.

tmcgill Over a year ago

The first way described above does not do what it says it does. It actually creates an array of size 5 (all elements undefined), assigning it to the variable a, and then immediately throws that array away and assigns to the variable a another, brand new array of size 5, with a zero stored at each index. To have kept the existing array and initialized its elements, what would have been needed was var a = new Array(5); followed by a[0] = 0; a[1] = 0; etc.

FrancescoMM Over a year ago

var a = [0, 0, 0, 0, 0];

kennebec · Accepted Answer · 2012-07-09 23:11:05Z

0

If you are creating an array whose main feature is it's length, rather than the value of each index, defining an array as var a=Array(length); is appropriate.

eg-

String.prototype.repeat= function(n){
    n= n || 1;
    return Array(n+1).join(this);
}

answered Jul 9, 2012 at 23:11

kennebec

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Comments

anshul · Accepted Answer · 2022-12-04 23:26:08Z

-1

You can declare like this

const [alice, bob] = [1, 2]

console.log(alice) // 1
console.log(bob) // 2

edited Dec 4, 2022 at 23:26

anshul

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answered Dec 1, 2022 at 16:01

LiamTran

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