JavaScript Functions

Definition: A function is a named, reusable block of code. Define it once, then call it whenever you need it — keeping code organised and avoiding repetition.

Example 1 — define and call

function greet() {
  console.log("Hello!");
}
greet();   // call it
greet();   // call again

Example 2 — parameters (inputs)

function greet(name) {
  console.log("Hello, " + name);
}
greet("Sam");
greet("Alex");

Example 3 — returning a value

function add(a, b) {
  return a + b;
}
let result = add(3, 4);
console.log(result);
console.log(add(10, 20));

Example 4 — arrow functions (modern short form)

const add = (a, b) => a + b;
const square = x => x * x;
console.log(add(10, 5));
console.log(square(6));

Example 5 — a small useful function

function isEven(n) {
  return n % 2 === 0;
}
console.log(isEven(4));  // true
console.log(isEven(7));  // false

💡 Tip: a good function does one clear job and has a descriptive name like calculateTotal.