Functions in JavaScript

Introduction

A function packages steps you can run by name, with parameters, many times. Functions are first-class values in JavaScript—you can pass them around, return them, and store them in variables. This chapter explains why functions matter and previews declaration styles used throughout the rest of the course.

Prerequisites

Why Functions Exist

Without functions, every script repeats the same blocks. Functions give you:

  • Reuse — one definition, many calls
  • Names — sendEmail() reads better than 40 anonymous lines
  • Testing — isolate behavior in small units
  • Abstraction — hide details behind a clear interface

Function Declaration

javascript
// Hoisted declaration
function greet(name) {
  return `Hello, ${name}!`;
}
 
console.log(greet("Ada"));

Function Expression

javascript
// Stored in a variable
const add = function (a, b) {
  return a + b;
};
 
console.log(add(2, 3));

Arrow Functions (Preview)

javascript
// Short syntax for expressions
const double = (n) => n * 2;
console.log(double(4));

Details in this and arrow functions.

Calling vs Defining

javascript
// define once
function logLine(message) {
  console.log(`[log] ${message}`);
}
 
// call many times
logLine("server started");
logLine("request received");

Parameters and Return Values

Functions accept arguments; return sends a value back (or undefined if omitted). See parameters chapter.

Functions as Values

javascript
// Pass function to another function
function runTwice(fn) {
  fn();
  fn();
}
 
runTwice(() => console.log("tick"));

Mini Example: Format Currency

javascript
// Reusable formatter
function formatUSD(amount) {
  return `$${amount.toFixed(2)}`;
}
 
console.log(formatUSD(19.5));
console.log(formatUSD(1000));

FAQ

Declaration vs expression?

Declarations are hoisted; expressions are not—order matters for const fn = function....

Do I need return?

Without return, the function yields undefined.

Are arrow functions always better?

No—use arrows for short callbacks; use function when you need this binding (see this and arrows).

What comes next?

Defining functions.