JavaScript: Object Basics

By Xah Lee. Date: . Last updated: .

JavaScript object is a collection of key/value pairs.

Each key/value pair of a object is called property. [see Property Overview]

Properties are unordered, and values can be changed anytime, and property can be added or removed, anytime.

Each object automatically inherit parent object's properties.

JavaScript objects are used for 2 purposes:

Create Object

Most easy way is to use Object Literal Expression, like this:

{name_1:val_1, name_2:val_2 etc}

// create a object with 3 properties
const nn = {"cat":19, "dog":20, "rabbit":25};

Access Property

2 syntax to access property:

// creating a object
const nn = {"cat":19, "dog":20, "rabbit":25};

// Accessing property, bracket notation
nn["cat"] // 19

// Accessing property, dot notation // 19

[see Property Dot Notation / Bracket Notation]

Add Property

Create a empty object, then add some properties:

const oj = {}; // creating a object
oj.c1 = 3;    // add a property c1
oj.c2 = 4;

console.log(oj); // { c1: 3, c2: 4 }

Change a Property's Value

const obj =  {"a": 1, "b": 2};
obj.a = 99;
console.log( obj); // { a: 99, b: 2 }

Delete a Property

To delete a property, use the delete [see Delete operator] .

const nn = {"cat":19, "dog":20, "rabbit":25};
console.log(nn); // { dog: 20, rabbit: 25 }

Enumerate Object's Properties

const nn = {"a":19, "c":20, "b":25};

// loop thru object's own enumarable properties
Object.keys(nn).forEach( (x => { console.log(x, nn[x]); }) );

// prints
// a 19
// c 20
// b 25


Nested Array and Object

Because object's property's value can be any object data type, so you can have any nested array and object. (because array is also a object. [see Understand JS Array])

// Key/Value, with one value being array:
const x = {"a":19, "b":20, "c":[3,4]};
console.log( x["c"][0]); // prints 3

Example of key/value with one value being a variable that eval to array:

const y = [3,4];
const x = {"a":19, "b":20, "c":y}; // the y is array
console.log( x["c"][0] ); // prints 3

The syntax for accessing elements can be chained.

// syntax for accesing array/hash can be chained
 {"a":19, "b":20, "c":[3,4]}["c"][0]
 // prints 3

Following is another example of JavaScript object. This is a JSON format.

// JSON format is basically nested js objects
const myStructure = {
  name: {
    first: "Cat",
    last: "bird"
  age: 19,
  hobbies: [ "shopping", "dancing" ]

JSON is essentially a (nested) JavaScript object (including array) where the leaf values are string or number.

Object and Methods

Remember, property names are string type or symbol type. [see Property Key]

Remember, property values can be any type, including any object. (function and array are both objects) [see Object Type]

Define Method

A method is just a property that has value of function.

j = {};                             // create a object
j.m = function (x) {return x + 1;}; // create a method named m

// calling the method
const y = j.m(3);

console.log(y);                 // prints 4


In JavaScript, when a function is called, it has a associated value called “ThisBinding”.

In function body, the “ThisBinding” is represented by the keyword this.

The purpose of “ThisBinding” is to allow function to implicitly work with a object. (without actually passing a argument in function call.)

When a function is called as a method in the form obj.f(), the value of “thisBinding” is the object obj.

// create a object, with one property p1
const obj = {"p1":3};

// make property p2 to be a function, which simply return 「this」
obj.p2 = function () { return this; };

// show obj
console.log( obj );               // { p1: 3, p2: [Function] }

console.log( obj === obj.p2() );   // true

// returns true because p2 is a function that returns 「this」, which is obj

How this keyword gets its value depends on many things. For detail, see this Binding

JavaScript Object Model

JavaScript object model is very different from Java, Python, Ruby.

For detail, see: Object Overview

Browser Console Object Syntax Error?

browser console js object syntax error 20161027
Common error when using brower console to evaluate a JavaScript object literal expression.

see Browser JS Console Object Syntax Error

JavaScript Object and Inheritance

JavaScript in Depth

JavaScript in Depth

Basic Syntax

Value Types




Object and Inheritance




Iterable 🌟