Functions

Why functions?

If you calculate GST in ten places and the rate changes, you must fix ten places. Put the calculation in one function, and you fix it once. Functions also give a name to an idea, which makes code easier to read.

Declaring and calling

function withGst(price, rate) {
  return price + (price * rate) / 100;
}

console.log(withGst(1000, 18)); // 1180
console.log(withGst(500, 5));   // 525

Parameters are the names in the brackets when you write the function. Arguments are the real values you pass when you call it. return sends a value back and ends the function at once. A function without return gives back undefined.

Default parameters

function greet(name = 'Guest') {
  return `Namaste, ${name}!`;
}
console.log(greet('Ravi')); // 'Namaste, Ravi!'
console.log(greet());       // 'Namaste, Guest!'

Arrow functions

An arrow function is a shorter way to write a function, very common in modern code. If the body is a single expression, you can skip the curly braces and the word return.

const square = (n) => n * n;
const add = (a, b) => {
  const sum = a + b;
  return sum;
};
console.log(square(4), add(2, 3)); // 16 5

Functions are values

You can store a function in a variable and pass it to another function. A function passed in this way is called a callback. You will use callbacks with array methods and events: "when the button is clicked, call this function".

Scope

Scope is where a variable can be used. A variable made with let or const inside a function or inside { } exists only there. This keeps functions independent: a total inside one function does not clash with a total in another.