JavaScript Functions

In this chapter, we will learn about JavaScript functions. Functions are fundamental building blocks in JavaScript, allowing you to encapsulate reusable code blocks. We will cover:

• What is a Function?
• Defining a Function
• Calling a Function
• Function Parameters and Arguments
• Return Statement
• Function Expressions
• Arrow Functions
• Higher-Order Functions
• Simple Programs using Functions

What is a Function?

A function is a block of code designed to perform a particular task. It is executed when "called" or "invoked". Functions help you organize your code, make it reusable, and improve readability.

Defining a Function

You can define a function using the function keyword followed by a name, parentheses (), and a block of code {}.

Syntax

function functionName(parameters) { // code to be executed } 

Example

function greet() { console.log("Hello, World!"); } 

In the example above, a function named greet is defined that prints "Hello, World!" to the console.

Calling a Function

You call a function by using its name followed by parentheses ().

Example

greet(); // Output: Hello, World! 

In the example above, the greet function is called, and it prints "Hello, World!" to the console.

Function Parameters and Arguments

Functions can take parameters (also called arguments) to perform operations using the input values.

Syntax

function functionName(parameter1, parameter2) { // code to be executed } 

Example

function add(a, b) { console.log(a + b); } add(5, 3); // Output: 8 

In the example above, the add function takes two parameters, a and b, and prints their sum. The function is called with arguments 5 and 3.

Return Statement

The return statement is used to return a value from a function. When a return statement is executed, the function stops executing.

Syntax

function functionName(parameters) { // code to be executed return value; } 

Example

function multiply(a, b) { return a * b; } let result = multiply(4, 5); console.log(result); // Output: 20 

In the example above, the multiply function returns the product of a and b. The result is stored in the result variable and printed to the console.

Function Expressions

A function expression defines a function as part of a larger expression syntax. Function expressions can be anonymous (without a name).

Example

let greet = function() { console.log("Hello, World!"); }; greet(); // Output: Hello, World! 

In the example above, an anonymous function is assigned to the variable greet, which can then be called like a regular function.

Arrow Functions

Arrow functions provide a shorter syntax for writing functions. They are always anonymous and are best suited for non-method functions.

Syntax

let functionName = (parameters) => { // code to be executed }; 

Example

let add = (a, b) => { return a + b; }; console.log(add(2, 3)); // Output: 5 

For simple expressions, you can omit the curly braces and the return keyword.

Example

let multiply = (a, b) => a * b; console.log(multiply(3, 4)); // Output: 12 

Higher-Order Functions

A higher-order function is a function that can take another function as an argument or return a function as a result. They are commonly used in functional programming.

Example

function applyOperation(a, b, operation) { return operation(a, b); } let sum = applyOperation(5, 10, (x, y) => x + y); console.log(sum); // Output: 15 let product = applyOperation(5, 10, (x, y) => x * y); console.log(product); // Output: 50 

In the example above, the applyOperation function takes two numbers and a function (operation) as arguments. It applies the operation to the numbers and returns the result.

Simple Programs using Functions

Program 1: Calculate the Area of a Rectangle

function calculateArea(length, width) { return length * width; } let area = calculateArea(5, 10); console.log("Area of the rectangle:", area); // Output: Area of the rectangle: 50 

Program 2: Check if a Number is Even or Odd

function isEven(number) { return number % 2 === 0; } let number = 7; if (isEven(number)) { console.log(number + " is even."); } else { console.log(number + " is odd."); } // Output: 7 is odd. 

Program 3: Find the Maximum of Three Numbers

function findMax(a, b, c) { return Math.max(a, b, c); } let max = findMax(10, 20, 15); console.log("The maximum number is:", max); // Output: The maximum number is: 20 

Program 4: Convert Celsius to Fahrenheit

function celsiusToFahrenheit(celsius) { return (celsius * 9/5) + 32; } let fahrenheit = celsiusToFahrenheit(25); console.log("25°C is", fahrenheit, "°F"); // Output: 25°C is 77°F 

Program 5: Generate a Random Number between a Range

function getRandomNumber(min, max) { return Math.floor(Math.random() * (max - min + 1)) + min; } let randomNum = getRandomNumber(1, 100); console.log("Random number between 1 and 100:", randomNum); 

Conclusion

In this chapter, you learned about JavaScript functions, including how to define, call, and use them with parameters and return values. We also explored function expressions, arrow functions, higher-order functions, and provided various use cases with simple programs to demonstrate the usage of functions. Understanding functions is crucial for writing modular and reusable code in JavaScript.