JavaScript: functions, parameters and scope

Inputs and output

Parameters name a function’s inputs; arguments are the values supplied when calling it. return supplies a result and ends that execution. console.log displays a value but does not return it. A function without an explicit return yields undefined.

function calculateTotal(price, quantity) {
  return price * quantity;
}
const total = calculateTotal(12, 3);
console.log(total); // 36

Block scope

Scope determines where a variable is available. let and const have block scope: a name declared inside braces is unavailable outside that block. Use them after declaration; accessing them before initialization can cause a ReferenceError.

Arrow functions

An arrow with no braces implicitly returns its expression. With braces, write return to supply a result. Arrows differ from traditional functions, including having no this binding of their own. Either form can work for simple calculations.

const double = (value) => value * 2;
const triple = (value) => {
  return value * 3;
};
console.log(double(4), triple(4)); // 8 12

One clear responsibility

calculateTotal only multiplies its inputs. Separating calculation from display makes it easier to check and reuse. Decide what types it accepts and how it should handle negative quantities or text: JavaScript can convert some values automatically.

Practice specific cases

1. Check calculateTotal(12, 0): it should return 0.
2. Write a function that converts minutes into seconds.
3. Try two inputs and verify the function does not accidentally depend on an outside variable.