Basics

Functions

Reusable blocks of code that take inputs and return a value.

1. Explanation

Functions encapsulate logic so it can be called many times. Declare with the function keyword or use arrow functions (=>). Functions can take parameters, return values, and have default parameter values. Arrow functions are shorter and do not have their own 'this'.

2. Syntax

function name(params) { return value; }
const name = (params) => value;

3. Example

Try it yourself

function add(a, b) {
  return a + b;
}

const square = (x) => x * x;

const greet = (name = "friend") => "Hello, " + name;

console.log(add(2, 3));     // 5
console.log(square(6));     // 36
console.log(greet());       // Hello, friend
console.log(greet("Aman")); // Hello, Aman

Console

Click Run to see output.

4. Expected output

output

5
36
Hello, friend
Hello, Aman

5. Common mistakes

✗Forgetting to return — the function returns undefined
✗Using arrow functions when you need a method-style 'this'
✗Mutating parameters instead of returning a new value

6. Practice questions

Write isEven(n) that returns true/false
Write max(a, b, c) that returns the largest
Write a function that converts Celsius to Fahrenheit

→ Try more practice problems