Where was prevScore defined?

Hey Stephen Pierce 👋 Great question. I'm assuming you're talking about this piece of code:

const Counter = () => {
    const [score, setScore] = React.useState(0);

    const incerementScore = () => {
        setScore(prevScore => prevScore +1);
    }
    ...
}

To understand what prevScore is lets first take a look at setScore. setScore is a function that updates the score state. You can pass setScore a value or a function (which is what we're doing here). We are passing setScore an arrow function prevScore => prevScore +1 where prevScore is the parameter the arrow function takes in (since it's just a parameter we can name it what ever we want, I just named it "prevScore"). Behind the scenes React uses this function to know how to update the score state. So it sets prevScore equal to the current value of the score state and sets score to score +1 (in our case above).

I hope this helps! If you want to learn more about useState's set function click this link.

If you want to learn why we can't just pass setScore(score +1) I recommend looking at this forum post