What’s the difference between “{}” and “[]” while

问题:

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={}

回答1:

Nobody seems to be explaining the difference between an array and an object.

[] is declaring an array.

{} is declaring an object.

An array has all the features of an object with additional features (you can think of an array like a sub-class of an object) where additional methods and capabilities are added in the Array sub-class. In fact, typeof [] === "object" to further show you that an array is an object.

The additional features consist of a magic .length property that keeps track of the number of items in the array and a whole slew of methods for operating on the array such as .push(), .pop(), .slice(), .splice(), etc... You can see a list of array methods here.

An object gives you the ability to associate a property name with a value as in:

var x = {};
x.foo = 3;
x["whatever"] = 10;
console.log(x.foo);      // shows 3
console.log(x.whatever); // shows 10

Object properties can be accessed either via the x.foo syntax or via the array-like syntax x["foo"]. The advantage of the latter syntax is that you can use a variable as the property name like x[myvar] and using the latter syntax, you can use property names that contain characters that Javascript won't allow in the x.foo syntax.

A property name can be any string value.

---

An array is an object so it has all the same capabilities of an object plus a bunch of additional features for managing an ordered, sequential list of numbered indexes starting from 0 and going up to some length. Arrays are typically used for an ordered list of items that are accessed by numerical index. And, because the array is ordered, there are lots of useful features to manage the order of the list .sort() or to add or remove things from the list.

回答2:

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);

回答3:

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'}

回答4:

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.

回答5:

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

回答6:

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

回答7:

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