Functions

Declaring, calling, and composing functions the foundation of reusable code.

Overview

A function is a named, reusable block of code. Instead of copying and pasting the same logic, you write it once in a function and call it whenever needed. Functions are first-class citizens in JavaScript they can be stored in variables, passed as arguments, and returned from other functions. There are two classic ways to define a function: a function declaration and a function expression. Declarations are hoisted you can call them before the line where they are defined in the file. Expressions assigned to a const are not hoisted. Functions accept parameters (the named placeholders in the definition) and receive arguments (the actual values passed when called). Every function returns a value if you omit return, the function returns undefined. Parameters can have default values, making the argument optional at the call site.

Key Points

•function declaration: function name() {} hoisted, callable before definition
•function expression: const name = function() {} not hoisted
•Parameters are the variables listed in the definition; arguments are the values passed in
•Default parameters: function greet(name = "World") used when argument is omitted or undefined
•Every function returns undefined unless you explicitly return a value
•Functions can return any value, including other functions

Examples

Declaration vs expression, parameters, return

// Declaration (hoisted)
function add(a, b) {
  return a + b;
}
console.log(add(3, 4)); // 7

// Expression (not hoisted)
const multiply = function(a, b) {
  return a * b;
};
console.log(multiply(3, 4)); // 12

Default parameters and optional arguments

function greet(name = "World", greeting = "Hello") {
  return `${greeting}, ${name}!`;
}

console.log(greet());              // Hello, World!
console.log(greet("Alice"));       // Hello, Alice!
console.log(greet("Bob", "Hi"));   // Hi, Bob!

Functions as values first-class citizens

function applyTwice(fn, value) {
  return fn(fn(value));
}

function double(x) {
  return x * 2;
}

console.log(applyTwice(double, 3)); // 12  (3 → 6 → 12)

Passing a function as an argument is called a callback. It's fundamental to arrays, events, and async code.

Common Mistakes

Calling a function without () greet logs the function itself, greet() calls it
Forgetting return the function implicitly returns undefined
Relying on hoisting for function expressions only declarations are hoisted

Functions

Overview

Key Points

Examples

Common Mistakes

Exercises

Knowledge Check

What is returned by a function that has no return statement?