Functions

What you'll learn

How to define a function, call it, pass values in with parameters and get a result out with return.

Why it matters

Functions let you name a piece of work and reuse it. They are the main way programs stay organised instead of becoming a long wall of code.

Explanation

A function is a named recipe. You define it once, then call it whenever you need it.

• name is a parameter: a placeholder for a value the caller will provide.
• The value you pass when calling, such as "Asha", is an argument.
• return sends a result back and ends the function.

A function that does not return gives back undefined.

The short arrow function form is common in modern code:

Example

Step-by-step walkthrough

1. add is defined with two parameters. Nothing runs yet.
2. add(2, 3) calls it: a is 2, b is 3, and return a + b sends back 5. That value is stored in total.
3. double(add(2, 3)) runs the inner call first (5) and then doubles it to 10.
4. describe(20) takes the first return, so it prints adult. The function ends right there.
5. describe(12) skips the if and reaches return "minor".

Try it yourself

Write a function square(n) that returns n * n, then print square(7).

Write a functionJavaScript

Editable codefunction square(n) { // return n times n } console.log(square(7)); // should print 49

Run ResetCopyCtrl+Enter to run

Common mistakes

• Forgetting return. The function runs but gives back undefined.
• Calling without parentheses. add is the function itself, add(2, 3) runs it.
• Code after return. It never runs.
• Using a variable defined inside a function from outside. It only exists inside.

Quick quiz

Summary

• function name(params) { return value; } defines reusable code.
• Arguments go in, return sends a result out.
• Arrow functions are a shorter way to write the same thing.

You finished the JavaScript beginner section. Next: arrays, objects and the DOM.

function greet(name) {
  return "Hello, " + name + "!";
}

const double = (n) => n * 2;

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

const total = add(2, 3);
console.log(total);          // 5

const double = (n) => n * 2;
console.log(double(add(2, 3)));  // 10

function describe(age) {
  if (age >= 18) {
    return "adult";
  }
  return "minor";
}
console.log(describe(20));   // adult
console.log(describe(12));   // minor