13. Objects

Objects provide a way to organise data and functionality. They are used to represent real-world entities, group related data together, and enable object-oriented programming.

In JavaScript an object is a collection of properties, where each property is a key-value pair. The values can be any type, including numbers, strings, arrays, functions, or even other objects.

13.1. Defining an Object

Syntax

You can define an object using curly braces {} containing a sequence of property and value pairs that represent the properties and methods. We will look at methods in a later section.

let objectName = {
    property1: value1,
    property2: value2,
    ...
    propertyN: valueN,
};

where

• objectName is the variable name holding the object

• property1 is the name of the first property

• value1 is the value associated with property1

Example

let person = {
    name: "Alice",
    age: 25,
    isStudent: true
};

console.log(person);

{ name: 'Alice', age: 25, isStudent: true }

13.2. Accessing Object Properties

You can access object properties using dot or bracket notation.

Dot Notation

console.log(person.name); // Prints: Alice

Bracket Notation

Use this when the property name is dynamic or contains special characters.

console.log(person["age"]); // Prints: 25

13.3. Adding and Modifying Properties

You can add properties to and modify the value of existing properties of an object.

Add a Property

person.city = "Sydney";
console.log(person); // { name: 'Alice', age: 25, isStudent: true, city: 'Sydney' }

Modify a Property

person.age = 26;
console.log(person.age); // Prints: 26

13.4. Methods in Objects

A method is a function stored as a property of an object.

Example

let car = {
    brand: "Toyota",
    start: function() {
        console.log("The car has started.");
    }
};

car.start();

The car has started.

13.5. Classes

A class is a template for creating objects. It defines properties and methods that the objects will have.

Syntax

class ClassName {
    constructor(parameter1, parameter2) {
        this.property1 = parameter1;
        this.property2 = parameter2;
    }

    method1() {
        // Code here
    }

    method2() {
        // Code here
    }
}

Where

• ClassName is the name of the class you are defining

• constructor is the method that is called when an instance of the class, i.e. an object, is created.

• parameters are a list of parameters passed to the constructor

• method1 and method2 are method definitions

Example

class Person {
    constructor(name, age) {
        this.name = name;
        this.age = age;
        this.alive = True
    }

    greet() {
        console.log(`Hello, my name is ${this.name}.`);
    }
}

let alice = new Person("Alice", 25);
console.log(alice.name); // Prints: Alice
alice.greet(); // Prints: Hello, my name is Alice.

13.6. Classes: A Thin Wrapper

In JavaScript, classes provide a way to create and organise objects, but at their core, they are essentially a thin wrapper around JavaScript’s built-in object and prototype system.

JavaScript has always been object-oriented, even before the introduction of the class keyword in ES6. Classes were introduced to make object creation and inheritance more readable and familiar for developers coming from other languages like Java or Python.

Best Practices

• Use plain objects for simple, ad-hoc data structures or when flexibility is required.

• Use classes when you need to enforce structure, create reusable blueprints, or handle complex inheritance.