Parameters, default values and return — Functions

Parameters and arguments

Parameters are the names that appear inside the parentheses when you define the function.
Arguments are the actual values you pass when you call it.

function multiply(a, b) { // a and b are parameters
  return a * b;
}
multiply(3, 4); // 3 and 4 are arguments

Default values

You can give a parameter a default value in case it is not provided:

function greet(name = "friend") {
  return "Hello, " + name;
}
console.log(greet());       // "Hello, friend"
console.log(greet("Lucia")); // "Hello, Lucia"

return

The keyword return returns a value and ends the function immediately.

⚠️ Classic trap: forgetting the return. A function without return always gives back undefined, no matter how many calculations it runs inside. If your function "returns nothing", check this first: it's the number-one mistake for beginners.

function isAdult(age) {
  if (age >= 18) {
    return true;  // exits here
  }
  return false;
}

Anything you write after an executed return never runs.

Examples

Calculating a discounted price: default value and early return

function finalPrice(price, discount = 0) {
  if (price <= 0) return 0;
  return price - price * discount;
}
console.log(finalPrice(100));      // 100
console.log(finalPrice(100, 0.2)); // 80