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
newkeyword) - Numbers can be objects (if defined with the
newkeyword) - Strings can be objects (if defined with the
newkeyword) - 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.
A primitive data type is data that has a primitive value.
JavaScript defines 5 types of primitive data types:
stringnumberbooleannullundefined
Primitive values are immutable (they are hardcoded and therefore cannot be changed).
if x = 3.14, you can change the value of x. But you cannot change the value of 3.14.
| Value | Type | Comment |
|---|---|---|
| “Hello” | string | “Hello” is always “Hello” |
| 3.14 | number | 3.14 is always 3.14 |
| true | boolean | true is always true |
| false | boolean | false is always false |
| null | null (object) | null is always null |
| undefined | undefined | undefined is always undefined |
Objects are Variables #
JavaScript variables can contain single values:
Example #
var person = "John Doe"; Objects are variables too. But objects can contain many values.
The values are written as name : value pairs (name and value separated by a colon).
Example #
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"}; A JavaScript object is a collection of named values
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:
- Define and create a single object, using an object literal.
- Define and create a single object, with the keyword
new. - Define an object constructor, and then create objects of the constructed type.
In ECMAScript 5, an object can also be created with the function 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 #
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
Spaces and line breaks are not important. An object definition can span multiple lines:
Example #
var person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
}; var person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue"; The two examples above do exactly the same. There is no need to use new Object().
For simplicity, readability and execution speed, use the first one (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:
var x = person; // This 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 #
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"}
var x = person;
x.age = 10; // This will change both x.age and person.age
