Functions & return

The idea

Walking into a side room (the function), doing some work with what you brought in, and carrying something back out with you (the return value) when you leave.

What it actually is

A function is a named block of code you can reuse any time you need it. It can take inputs (called parameters), and it can hand a value back using return. Forget return, and the function just quietly gives back nothing (undefined).

How you write it

function name(parameter) {
  return value;
}

A worked example

function doubleScore(points) {
  return points * 2;
}
console.log(doubleScore(10));

doubleScore takes one input, points. Calling doubleScore(10) runs the function with points set to 10, which computes 10 * 2 and hands 20 back out - console.log then prints that returned value.

function greetPlayer(name) {
  return `Welcome, ${name}!`;
}
console.log(greetPlayer("Zoe"));

Functions can return text too, not just numbers - here it builds and hands back a whole greeting sentence using the name that was passed in.

Mistakes children actually make

Mistake 1: forgetting return - a function without it still runs, but silently gives back undefined instead of the value you expected, even if it printed something with console.log along the way. Mistake 2: mixing up console.log and return - console.log only shows something on screen, it doesn't hand a value back to the code that called the function. Mistake 3: calling a function with the wrong number of arguments - addBonus(50) when addBonus expects TWO parameters doesn't crash, it just quietly sets the missing one (bonus) to undefined, and any math with it becomes NaN instead of a real number.

Then they try it

In the app this lesson continues with the animated explanation, spoken aloud, and then the practice: Complete addBonus so it returns score + bonus, then call addBonus(50, 25) and print the result. The editor runs your child’s real code and checks the result, with their coding buddy reacting to what they wrote.