未定义的变量“ num”如何正常工作？

Question

我为“ hang子手”游戏编写了代码来练习JS编码（目前非常简单）。我忘了初始化变量'num'...我只是写了num = 2而...它可以工作，但是我不明白如何？

app.js

num = 2
document.addEventListener('click', guesser.guessLetter);

guesser.js

const guesser = (() => {
    return {
        guessLetter: (e) => {
            let picture = document.querySelector('.picture')

            console.log(e.target)
            guess = e.path[0]

            if (guess.className === 'letter') {
                if (guess.id === 'h' || guess.id === 'a' || guess.id === 'n' || guess.id === 'g' || guess.id === 'm') {
                    let newLetter = document.querySelectorAll(`#${guess.id}1`);
                    let usedLetter = document.getElementById(`${guess.id}`)
                    newLetter.forEach(element => {
                        element.src = `./static/pic/chalk-alphabet/${guess.id}.png`
                })

                    usedLetter.src = `./static/pic/chalk-alphabet/blank.png`
            } else {

                    picture.src = `./static/pic/hangman${num}.jpg`

                    if (num < 10) {
                        return num = num + 1
                    } else {
                        return num = 10
                    }
                }
            }
        }
    }
}
)()