JavaScript Functions

Declaring a function

A function is a reusable block of code. Declare it with function, then call it with parentheses:

function greet(name) {
  console.log(`Hello, ${name}!`);
}
greet("Ada");
greet("Linus");

Hello, Ada!
Hello, Linus!

Return values

return sends a value back to the caller and ends the function. A function without return returns undefined.

function add(a, b) {
  return a + b;
}
function logOnly(x) {
  console.log("got", x);
}
const sum = add(2, 3);
console.log(sum);
console.log(logOnly(1));

5
got 1
undefined

Function expressions

Functions are values: you can store them in variables, pass them to other functions and return them.

const square = function (n) {
  return n * n;
};
function applyTwice(fn, value) {
  return fn(fn(value));
}
console.log(square(4));
console.log(applyTwice(square, 3));

16
81

Function declarations are hoisted — you can call them before the line where they're defined. Function expressions assigned to const are not.