无法对 javascript 中的 for 循环内的值求和

Question

I believe this is a scope problem, but idk why it's not working. I'm fairly new to js, tried checking the documentation but still couldn't solve it

For some reason, the "soma" variable always logs as "NaN" when inside the for loop. Prob is something simple, but i've been hitting my head on the desk for almost an hour now. Help D:

 const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8] // Luhn Algorithm validateCred = (array) => { var count = 0 var soma = 0 for (i=array.length; i;= -1. i--) { if (count === 0) { count += 1 soma += array[i] } else { if(array[i]*2 > 9) { soma += array[i]*2 - 9 } else { soma += array[i]*2 } count = 0 } console.log(soma%10) // expected output === 0 } } validateCred(valid1)

Answer 1

Change the i=array.length to i=array.length - 1 & move your console.log after loop

const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8]
// Luhn Algorithm
validateCred = (array) => {
    var count = 0
    var soma = 0
    for (i=array.length - 1; i != -1; i--) {
        if (count === 0) {
            count += 1
            soma += array[i]
        } else {
            if(array[i]*2 > 9) {
                soma += array[i]*2 - 9
            } else {
                soma += array[i]*2
            }
            count = 0
        }
    }
     console.log(soma%10) // expected output === 0
}

validateCred(valid1)

Answer 2

The last item of an array is in the array.length-1 position, since the first element is in the 0 position that's why array[array.length] return nan.Besides you may want to read about the standard methods for iterate over an array: array.foreach(), array.filter(), array.reduce and array.map().