javascript - When I call this recursive function with --> multiply( [ 1 , 2 , 3 , 4 , 5 ] , 3 ) , I get NaN. What is the reason behind?

Question

When I call this recursive function with --> multiply( [ 1 , 2 , 3 , 4 , 5 ] , 3 ) , I get NaN. What is the reason behind?

let multiply = function (arr, num) {
    if (num <= 0) {
        return 1;
    }
    return arr[num - 1] * multiply([1, 2, 3, 4, 5], num - 1);
}
console.log(multiply( [ 1 , 2 , 3 , 4 , 5 ] , 3 ));

Answer 1

Because in the last call where num = 0 you are trying to access arr[num-1] which is arr[0-1] => arr[-1] = undefined. Hence all the result becomes NaN.

You need to write the method in following way,

let multiply = function (arr, num) {
   
    if (num < 0) {
        return 1;
    }
   
    return arr[num] * multiply([1, 2, 3, 4], num - 1);
}

console.log(multiply( [ 1 , 2 , 3 , 4 , 5 ] , 3 ));