What does [] mean in JavaScript?

Question

In the following javascript code there is [] being assigned as the value of a variable, what does it mean?

var openTollDebug = [];

Solution 1

it is an array literal. It is not quite the same as declaring new Array() - the Array object can be overwritten in JavaScript, but the array literal can't. Here's an example to demonstrate

// let's overwrite the Array object
Array = function(id) {
 this.id = id;
}

var a = new Array(1);
var b = [];

console.log(a.hasOwnProperty("id")); // true
console.log(b.hasOwnProperty("id")); // false

console.log(a.push); // false, push doesn't exist on a
console.log(b.push); // true,  but it does on b

b.push(2);
console.log(b); // outputs [2]

Solution 2

It means an array.

var openTollDebug = [];

declares the openTollDebug variable and initializes it to an empty array. To put elements into the array you could do the following:

var stringArray = ['element1', 'element2', 'element3'];
alert(stringArray[1]); // displays 'element2'
var numberArray = [1, 2, 3, 4];
alert(numberArray[2]); // displays 3
var objectArray = [{ name: 'john' }, { name: 'peter' }, { name: 'tom' }];
alert(objectArray[1].name); // displays 'peter'

Solution 3

It's an empty array, and is equal to

var openTollDebug = new Array();

Solution 4

It is shorthand for empty array. Same as new Array(). Also {} is an empty object. Objects are like hashtables in Js so you can use it as a dictionary.

Solution 5

It creates an empty array.
This is a good way to have a non-null object.
In JavaScript, it is then very easy to add functions and properties to that object. For example:

openTollDebug.title = 'hello world';
openTollDebug.show = function(){alert('Debug');};

As an array, you can add items:

openTollDebug.push('added item');
openTollDebug[3] = 'just add anywhere';

View more solutions

Comments

[Tim Down]

So long as Array hasn't been overwritten, new Array() and [] are functionally identical.

[Russ Cam]

@Tim - yes they are. There is also a difference in intializing values and length - new Array(5) is not the same as [5] :)

[Russ Cam]

for the person who gave the -1 : why? I believe I qualified that declaring [] and new Array() is not quite the same as the latter is based on the condition that Array has not been overwritten.

[Quaternion]

You should check to see if there are answers which already provide the same information.

[Pacerier]

@RussCam, So how do we override [] constructor?