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JavaScript Objects



In JavaScript, objects are king. If you understand objects, you understand JavaScript.


In JavaScript, almost "everything" is an object.

Booleans can be objects (if defined with the new keyword)
Numbers can be objects (if defined with the new keyword)
Strings can be objects (if defined with the new keyword)
Dates are always objects
Maths are always objects
Regular expressions are always objects
Arrays are always objects
Functions are always objects
Objects are always objects

All JavaScript values, except primitives, are objects.

JavaScript Primitives
A primitive value is a value that has no properties or methods.
3.14 is a primitive value
A primitive data type is data that has a primitive value.
JavaScript defines 7 types of primitive data types:
Examples

string
number
boolean
null
undefined
symbol
bigint


Immutable
Primitive values are immutable (they are hardcoded and cannot be changed).
if x = 3.14, you can change the value of x, but you cannot change the value of 3.14.


ValueTypeComment
"Hello"string"Hello" is always "Hello"
3.14number3.14 is always 3.14
truebooleantrue is always true
falsebooleanfalse is always false
nullnull (object)null is always null
undefinedundefinedundefined is always undefined


Objects are Variables
JavaScript variables can contain single values:

Example

let person = "John Doe";

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JavaScript variables can also contain many values.
Objects are variables too. But objects can contain many
values.
Object values are written as name : value pairs (name and value separated by a
colon).

Example

let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

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A JavaScript object is a collection of named values

It is a common practice to declare objects with the const keyword.

Example

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

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Object Properties
The named values, in JavaScript objects, are called properties.


Property
Value


firstName
John


lastName
Doe


age
50


eyeColor
blue


Objects written as name value pairs are similar to:

Associative arrays in PHP
Dictionaries in Python
Hash tables in C
Hash maps in Java
Hashes in Ruby and Perl


Object Methods
Methods are actions that can be performed on objects.
Object properties can be both primitive values, other objects, and functions.
An object method is an object property containing a function
definition.


Property
Value


firstName
John


lastName
Doe


age
50


eyeColor
blue


fullName
function() {return this.firstName + " " + this.lastName;}



JavaScript objects are containers for named values, called properties and methods.

You will learn more about methods in the next chapters.

Creating a JavaScript Object
With JavaScript, you can define and create your own objects.
There are different ways to create new objects:

Create a single object, using an object literal.
Create a single object, with the keyword new.
Define an object constructor, and then create objects of the constructed type.
Create an object using Object.create().


Using an Object Literal
This is the easiest way to create a JavaScript Object.
Using an object literal, you both define and create an object in one
statement.
An object literal is a list of name:value pairs (like age:50) inside curly braces {}.
The following example creates a new JavaScript object with four properties:

Example

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

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Spaces and line breaks are not important. An object definition can span multiple lines:

Example

const person = {  firstName: "John",  lastName: "Doe",  age: 50,  eyeColor: "blue"};

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This example creates an empty JavaScript object,
and then adds 4 properties:

Example

const person = {};
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";

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Using the JavaScript Keyword new
The following example create a new JavaScript object
using new Object(),
and then adds 4 properties:

Example

const person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";

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The examples above do exactly the same.
But there is no need to use new Object().
For readability, simplicity and execution speed, use the object literal method.


JavaScript Objects are Mutable
Objects are mutable: They are addressed by reference, not by value.
If person is an object, the following statement will not create a copy of person:


const x = person;  // Will not create a copy of person.


The object x is not a copy of person. It is
person.
Both x and person are the same object.
Any changes to x will also change person, because x and person are the same object.

Example

const person = {
  firstName:"John",
  lastName:"Doe",
  age:50, eyeColor:"blue"
}
const x = person;
x.age = 10;      // Will change both x.age and person.age

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Reference: https://www.w3schools.com/js/js_object_definition.asp