Function Intro

Functions are reusable blocks of code designed to perform a particular task. They are fundamental to every programming language. This lesson covers how to write them: the keyword, the name, the parentheses, and the body. Functions help you reuse code (write once, run many times), organize code into smaller parts, and make code easier to read and fix.

// Syntax

function name(p1, p2, ...) {
  // code to be executed
}

• The function keyword starts the definition
• followed by the function's name, which follows the same rules as variable names
• followed by parentheses ( ), holding optional parameters separated by commas
• followed by curly braces { }, holding the body: the code to run

A function can also send an optional value back to whoever called it, with return.

function sayHello() {
  console.log("Hello!");
}

This one has no parameters (empty parentheses) because it needs no input. Very short functions are sometimes written on one line, function sayHello() { return "Hello World"; }, but spreading them over several lines is more common and easier to read.

// Defining Does Not Run It

Writing a function only defines it. The code above does nothing on its own. To run it, call it: its name followed by parentheses. The () means "execute now":

function sayHello() {
  console.log("Hello!");
}

sayHello();
sayHello();

// Naming Functions

Function names follow the same rules as variable names (letters, digits, _, $; no leading digit) and use camelCase. Because functions do things, start their names with a verb:

// Good: clear actions
calculateTotal()
showMenu()
getUserName()
isValidEmail()    // "is" / "has" / "can" for true/false answers

// Vague: what do these do?
stuff()
doIt()
data()

// A Function with Input

Put a name inside the parentheses to accept a value. Inside the body it works like a variable:

function square(n) {
  console.log(n * n);
}

square(4);
square(9);

Separate several parameters with commas: function multiply(a, b) { return a * b; }.

// A Function That Gives Back a Value

Printing is fine for demos, but usually you want the result so you can use it. return hands it back to whoever called the function, and the returned value can be stored in a variable:

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

let sum1 = add(5, 5);
let sum2 = add(50, 50);
console.log(sum1, sum2);

The same code with different input produces different results. That is the whole point.

// Using a Function Like a Value

A call to a function that returns something can go anywhere a value can: in math, in a string, in a condition. You do not always need a variable in between:

function toCelsius(fahrenheit) {
  return (5 / 9) * (fahrenheit - 32);
}

// With a variable in between
let x = toCelsius(77);
let text1 = "The temperature is " + x + " Celsius";

// Using the call directly
let text2 = "The temperature is " + toCelsius(77) + " Celsius";

console.log(text2);

// Input and Output

The most useful functions take something in and give something back:

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

// Local Variables

Variables declared inside a function are local to it. They are created when the function starts, deleted when it finishes, and can only be used from inside it:

// code here can NOT use carName

function myFunction() {
  let carName = "Volvo";
  // code here CAN use carName
}

// code here can NOT use carName
console.log(carName);

// Functions Calling Functions

function square(n) {
  return n * n;
}

function hypotenuse(a, b) {
  return Math.sqrt(square(a) + square(b));
}

console.log(hypotenuse(3, 4));

Small functions that each do one job can be combined into bigger ones. This is how large programs are built.

// Quick Check

function double(n) {
  return n * 2;
}
console.log(double(3) + double(4));

function setup() {
  let level = 1;
}
setup();
console.log(level);

// Try It

▶ Open Playground

1. Click Open Playground. Finish showBanner() so it prints a line of 20 dashes, then call it three times
2. Write double(n) that returns n times 2, and print double(21)
3. Add square(n), then write cube(n) that uses square(n) inside it
4. Add toCelsius() and use it directly inside a template string to print the temperature for 212°F