[javascript] What’s the difference between “{}” and “[]” while declaring a JavaScript array?

What’s the difference between “{}” and “[]” while declaring a JavaScript array? Normally I declare like

var a=[];

What is the meaning of declaring the array as var a={}

var a = [];

it is use for brackets for an array of simple values. eg.

var name=["a","b","c"]

var a={}

is use for value arrays and objects/properties also. eg.

var programmer = { 'name':'special', 'url':'www.google.com'}

they are two different things..

[] is declaring an Array:
given, a list of elements held by numeric index.

{} is declaring a new object:
given, an object with fields with Names and type+value,
some like to think of it as "Associative Array". but are not arrays, in their representation.

You can read more @ This Article

Syntax of JSON

object = {} | { members }

• members = pair | pair, members
• pair = string : value

array = [] | [ elements ]

• elements = value | value elements

value = string|number|object|array|true|false|null

When you declare

var a=[];

you are declaring a empty array.

But when you are declaring

var a={};

you are declaring a Object .

Although Array is also Object in Javascript but it is numeric key paired values. Which have all the functionality of object but Added some few method of Array like Push,Splice,Length and so on.

So if you want Some values where you need to use numeric keys use Array. else use object. you can Create object like:

var a={name:"abc",age:"14"}; 

And can access values like

console.log(a.name);

It can be understood like this:

var a= []; //creates a new empty array
var a= {}; //creates a new empty object

You can also understand that

var a = {}; is equivalent to var a= new Object();

Note:

You can use Arrays when you are bothered about the order of elements(of same type) in your collection else you can use objects. In objects the order is not guaranteed.

[ ] - this is used whenever we are declaring an empty array,

{ } - this is used whenever we declare an empty object

typeof([ ]) //object

typeof({ }) //object

but if your run

[ ].constructor.name  //Array

so from this, you will understand it is an array here Array is the name of the base class. The JavaScript Array class is a global object that is used in the construction of arrays which are high-level, list-like objects.

In JavaScript Arrays and Objects are actually very similar, although on the outside they can look a bit different.

For an array:

var array = [];
array[0] = "hello";
array[1] = 5498;
array[536] = new Date();

As you can see arrays in JavaScript can be sparse (valid indicies don't have to be consecutive) and they can contain any type of variable! That's pretty convenient.

But as we all know JavaScript is strange, so here are some weird bits:

array["0"] === "hello"; // This is true
array["hi"]; // undefined
array["hi"] = "weird"; // works but does not save any data to array
array["hi"]; // still undefined!

This is because everything in JavaScript is an Object (which is why you can also create an array using new Array()). As a result every index in an array is turned into a string and then stored in an object, so an array is just an object that doesn't allow anyone to store anything with a key that isn't a positive integer.

So what are Objects?

Objects in JavaScript are just like arrays but the "index" can be any string.

var object = {};
object[0] = "hello"; // OK
object["hi"] = "not weird"; // OK

You can even opt to not use the square brackets when working with objects!

console.log(object.hi); // Prints 'not weird'
object.hi = "overwriting 'not weird'";

You can go even further and define objects like so:

var newObject = {
    a: 2,
};
newObject.a === 2; // true