Functions

Functions are the unit of reuse (the decomposition fundamentals lesson). JavaScript has two main ways to write them, and you'll read both constantly.

Two ways to write a function

A declaration uses the function keyword; an arrow function is a shorter form assigned to a variable:

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

const addArrow = (a, b) => {
  return a + b;
};

// for a single expression, an arrow can be even shorter — the result is returned:
const addShort = (a, b) => a + b;

All three compute the same thing. Arrow functions are popular for short callbacks; declarations read well for named, standalone logic. (They differ in subtle ways around this, which matters later with classes — for now, treat them as two spellings.) When a function captures variables from its surrounding scope it forms a closure.

Parameters, defaults, return

Parameters can have defaults, and return hands a value back:

function greet(name, greeting = "Hello") {
  return greeting + ", " + name + "!";
}
greet("Ada");            // "Hello, Ada!"
greet("Ada", "Hi");      // "Hi, Ada!"

A function with no return (or a bare return) gives back undefined.

Functions are values

As you saw with binding, functions are ordinary values — you can store them and pass them to other functions. This is what makes array methods like map work:

const nums = [1, 2, 3];
const doubled = nums.map((n) => n * 2);   // [2, 4, 6]

map takes a function and calls it on each element — passing behaviour as data.

Where to go next

Next: control flow — conditionals and loops to drive your functions.