What is a Function in JavaScript? | Function Examples & Guide

Functions in JavaScript. Learn what functions are in JavaScript with examples. This beginner-friendly guide covers function declarations, parameters, return values, and more.

✅ Function Declaration and Function Call

Here’s how you define (declare) a simple function and then call it:

function sayHello() {

  console.log('Hello, Sagar!')

}

sayHello() // Function call

Explanation:

• function sayHello() is the function declaration.

• sayHello(); is the function call.

🧠 Functions with Parameters

You can also make your functions dynamic by using parameters.

JavaScript

function sayHello(name) {
  console.log('Hello, ' + name)
}

sayHello('Sagar') // Output: Hello, Sagar
sayHello('John') // Output: Hello, John

Why use parameters?

Parameters allow you to pass data into functions, making them reusable for different values.

➕ Function Example: Adding Two Numbers

Let’s look at another example where a function takes two parameters and adds them.

JavaScript

function sum(a, b) {
  let add = a + b
  console.log(add)
}

let x = 10
let y = 20
let z = 30

sum(x, y) // Output: 30
sum(y, z) // Output: 50

🔁 Returning Values from Functions

Functions can also return values using the return statement. This allows the result to be stored or used elsewhere in your code.

JavaScript

function square(x) {
  let result = x * x
  return result
}

let value = square(-3)
console.log(value) // Output: 9

🧠 Why return values?

Returning values makes your functions more flexible and allows you to store results for later use or use them in expressions.

🗳️ Function Example: Check Voting Eligibility

Here’s a real-world example that checks if a person is eligible to vote based on their age.

JavaScript

function eligibleToVote(age) {
  if (age < 0) {
    console.log('Invalid age')
  } else if (age >= 18) {
    console.log('User is eligible to vote')
  } else {
    console.log('User is not eligible to vote')
  }
}

eligibleToVote(0) // Output: Invalid age
eligibleToVote(5) // Output: User is not eligible to vote
eligibleToVote(17.7) // Output: User is not eligible to vote
eligibleToVote(19) // Output: User is eligible to vote

✅ Practical Use Case:

Great for validating user input in form fields before submission.

🔢 Function Example: Check Even or Odd Number

This function determines whether a given number is even or odd.

JavaScript

function evenOrOdd(num) {
  if (num % 2 === 0) {
    console.log('Even number')
  } else {
    console.log('Odd number')
  }
}

evenOrOdd(2) // Output: Even number
evenOrOdd(3) // Output: Odd number
evenOrOdd(10) // Output: Even number
evenOrOdd(17) // Output: Odd number

💡 When to Use:

This logic is commonly used in loops, conditions, and validations.

📝 Conclusion

JavaScript functions are essential for writing clean, reusable, and modular code. In this tutorial, you learned:

• What a function is

• How to declare and call functions

• Using parameters to make functions dynamic

• Returning values from functions

By mastering functions, you lay the foundation for writing more efficient and powerful JavaScript programs.

📚 Also Learn and Explore...

• JavaScript function tutorial

• What is a function in JavaScript

• How to declare functions in JavaScript

• JavaScript function examples

• Function with parameters in JavaScript

• Returning values from JavaScript functions

Happy coding!

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