Decrement the value of a variable at every function call

I am creating a slot machine program using JavaScript, and want to decrement the value of coins at every function call. For example, coins = 20. It should decrement after every time the casino() function is called. Basically the function can be called as many times until coins = 0.
Is this a situation where the application of let vs var comes into play?
Here is a snippet of my code:

let reel1 = ["cherry", "lemon", "apple", "lemon", "banana", "banana", "lemon", "lemon"]
let reel2 = ["lemon", "apple", "lemon", "lemon", "cherry", "apple", "banana", "lemon"]
let reel3 = ["lemon", "apple", "lemon", "apple", "cherry", "lemon", "banana", "lemon"]
let slotReel = []
let coins = 20

function casino(r1, r2, r3) {
    coins = coins - 1
    let random1 = reel1[Math.floor(Math.random() * reel1.length)]
    let random2 = reel2[Math.floor(Math.random() * reel3.length)]
    let random3 = reel3[Math.floor(Math.random() * reel3.length)]
    slotReel.push(random1, random2, random3)
    //.....some logic
}

casino(reel1, reel2, reel3)