JavaScript Function Declaration

A function declaration defines a named function. It consists of the function keyword, the name of the function, a list of parameters, and the function body.

Syntax:

function functionName(parameter1, parameter2, /* ... */) {
  // Code to be executed when the function is called
}

• functionName: The name of the function.
• parameter1, parameter2, ...: The parameters that the function accepts (if any).
• Function body: The code to be executed when the function is called.

Why it is Used:

• Modular Code: Encapsulates a set of operations into a reusable and modular unit.
• Code Organization: Enhances code organization and readability by grouping related functionality.
• Code Reusability: Enables the reuse of the same set of operations in different parts of the code.

Example:

// Function Declaration
function greet(name) {
  console.log('Hello, ' + name + '!');
}

// Calling the Function
greet('John'); // Output: Hello, John!

 

Examples:

Simple Addition Function

// Function Declaration for Addition
function addNumbers(a, b) {
  return a + b;
}

// Calling the Function
const sum = addNumbers(5, 7);
console.log('Sum:', sum); // Output: Sum: 12

Greeting Function with Default Parameter

// Function Declaration with Default Parameter
function greet(name = 'Guest') {
  console.log('Hello, ' + name + '!');
}

// Calling the Function
greet(); // Output: Hello, Guest!
greet('Alice'); // Output: Hello, Alice!

Area of a Rectangle Function

// Function Declaration for Calculating Area
function calculateRectangleArea(length, width) {
  return length * width;
}

// Calling the Function
const area = calculateRectangleArea(10, 5);
console.log('Area of Rectangle:', area); // Output: Area of Rectangle: 50

Checking Even or Odd Function

// Function Declaration for Even or Odd
function checkEvenOrOdd(number) {
  return number % 2 === 0 ? 'Even' : 'Odd';
}

// Calling the Function
console.log(checkEvenOrOdd(7)); // Output: Odd
console.log(checkEvenOrOdd(10)); // Output: Even

 

Summary

• Function declarations define named functions in JavaScript.
• They consist of the function keyword, a name, parameters, and a function body.
• Functions declared using this syntax are hoisted, meaning they can be called before their actual declaration in the code.
• Function declarations are a fundamental building block for creating modular and reusable code in JavaScript.