function is like a mini-program within a program, designed to be reusable

this is used so that you don't have to repeat code in a repetitive task

syntax

function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("Alice"); // Output: Hello, Alice!

function expression

with name as a parameter and Bob as an argument

const hello = function (name) {
  console.log("Hello, " + name + "!");
};

hello("Bob"); // Output: Hello, Bob!

function with return value

function multiply(num1, num2) { 
  return num1 * num2;
}

const result = multiply(5, 3); 
console.log(result); // Output: 15

arrow function

const multiply = (num1, num2) => {
  return num1 * num2;
}

const result = multiply(5, 5);

console.log(result); // Output: 25

arrow function one-line code

because it doesn't have a parameter we can leave () empty

because it's one line it's allowed to not use {} after the =>

const world = () => console.log('hello world!');

world() // Output: hello world!

arrow function one-line code with return

because it has only 1 parameter it's allowed to not use ()

because it's one-line it's using () as a return

const double = num => (num * 2)

const result = double(5)

console.log(result); // Output: 10