Summary

In this lesson, you learned that:

• A function is a named piece of code that you write once and can run as many times as you need. Thanks to functions, you don't have to repeat code, and when you change a function, the change applies everywhere it's used.
• You define a function with the keyword function, a name, parentheses with parameters and a body in curly braces: function printArea(width, height) { ... }. A definition alone doesn't run anything.
• You call a function with its name and parentheses: printArea(5, 6);. It's good practice to define functions first and call them below.
• A parameter is a name in the definition, an argument is a value in the call. Arguments go into parameters in order. A parameter that gets no argument is undefined, unless it has a default value: function printPrice(price, currency = "EUR").
• return gives a value back to the place of the call, so you can use the call like a value: console.log(add(2, 3)), const total = add(2, 3) * 2;. console.log only shows a value. A function without return returns undefined.
• return ends the function right away, and the rest of the function doesn't run. With conditions, the first return that runs wins.
• Variables created inside a function, and its parameters, exist only inside that function: they are local. Variables from outside can be used inside functions. To get a value out of a function, return it.
• A function can take an array, go through it with a loop and return a result. The return goes after the loop.
• Arrow functions are a shorter way to write functions: const add = (a, b) => a + b; returns the value after the arrow. With a body in curly braces, you need return: const area = (width, height) => { ... return ...; };.

In the next lesson, Objects, you will learn to keep values that belong together, such as a person's name and age, in a single value.