🧱 Objects

Everything you need to know about the JavaScript Object

The JavaScript object is a collection of key-value pairs, similar to a map, dictionary, or hash-table in other programming languages. Anything that is not a JS primitive is an Object.

  • An Object is a collection of properties.
  • A Property is a key-value pair that contains a name and a value.
  • A Property Name is a unique value that can be coerced to a string that points to a value.
  • A Property Value can be any value, including other objects or functions, that associated with the name/key.
An object is a collection of properties, aka key-value pairs

Object Basics


Create an empty object. You have several options.

file_type_js objects.js
// literal
const dog = { }

// constructor
const cat = new Object();

// static method
const horse = Object.create({ })

Get and Set Properties

Now that we have an empty object, we need to add properties to it using accessors.

Valid property names include anything that can be coerced to a string, but must not used reserved words like function, var, return and so on.

get = object.property;
object.property = set;

A potential gotcha with dot notation is that you can only access names that follow variable name identifier conventions, i.e without spaces or that do not start with a digit.

obj['hi mom'] = 1;
obj[23] = 1;

// syntax errors
obj.hi mom

Since ES6, we have a convenient shorthand for setting properties:

let hello;
let world;

// Old way 💩
const obj = {
    hello: hello,
    world: world

// Modern way 👍
const obj = {

Use a variable or expression as a property name by wrapping it in brackets - this is called a computed property.

const x = 'howdy';

const obj = {
  [x]: 23

obj.howdy // 23

Object properties can be removed with the delete keyword.

delete obj.hello;
delete obj.world;


An object is stored in the heap memory, which means variables maintain a reference to it, as opposed to a full copy of it. When checking for object equality, it checks the reference - not the actual value of properties.

const original = { }

const x = original;
const y = original;

x === y; // true
x === {}; // false

Any variable that points to that reference can set its properties and they will be shared between all variables.

x.hello = 'world';

original.hello; // world
y.hello; // world

Combine Objects

But what if we want to clone an object to create a separate reference? Object.assign allows us to copy an object’s properties and create a new reference. Its properties will be copied to the new object, thus changes to the original object will not affect the clone.

const original = {
    hello: 'world'

const clone = Object.assign({ }, original);

clone === original; // false

original.hello = 'changed!';

clone.hello; // world (did not change)

Spread Syntax

A more concise alternative to Object.assign is the spread syntax.

const clone = Object.assign({ }, original);

const sugar = { ...original };

const sugar = { ...original, hola: 'mundo' }; 

Object Methods

When a function is assigned to an object, it is called a method.


const obj = {
  hello() {



In a normal method, this refers to the object on which it is defined.

const obj = {
  username: 'Jeff',
  hello() {
	console.log(`My name is ${this.username}`)

obj.hello(); // My name is Jeff


Functions using the arrow syntax are not bound to this, so it refers to the outer or global this context.

const obj = {
  username: 'Jeff',
  hello: () => console.log(this.username)

obj.hello(); // My name is undefined


In certain JS libraries you will see method chaining with obj.doThis().toThat(), which is made possible by simply returning the value of this from each method.

file_type_js jquery.js
const game = {
  hitpoints: 100,
  log() {
    console.log(`👾 ${this.hitpoints}`);
  takeDamage() {
    this.hitpoints -= 10;
    return this; // Required for chaining
  heal() {
    this.hitpoints += 10;
    return this; // Required for chaining


👾 90
👾 80
👾 70
👾 80


Constructors are just functions that describe how to create an Object.

function Boat(name) {
  this.name = name;
  this.created = Date.now()

  this.horn = function () {

The object is then instantiated with the new keyword.

const sally = new Boat('Sally');
const molly = new Boat('Molly');

sally.horn() // Sally

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