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up vote 0 down vote favorite

I saw this javascript snippet/ question here The best answer is 

function sumDigits(number) {
    var remainder = number % 10;
    var sum = remainder;
    if(number >= 10) {
        var rest = Math.floor(number / 10);
        sum += sumDigits(rest); 
    }
    return sum;
}

I'm trying to understand how it works. I can understand some of the lines but it's confusing though.

What I can understand is 

var remainder = number % 10;
var sum = remainder;

return the remainder of the number. In other words, if the number is 145 and it's divided by 10, the remainder is 5. Therefore, the sum is 5. Next,

if(number >= 10) {

The number needs to be more than or equal to 10 because you cannot add up a single digit by itself, therefore '10' is the minimum in which you can add two digits together. Next,

var rest = Math.floor(number / 10);

145 divided by 10 is 14.5. Math.floor will round it down to 14. Therefore, rest = 14. Next,

sum += sumDigits(rest);

Since rest is 14, that means that sumDigits is 14. So since the sum is 5, it adds itself to the rest. In other words, sum = 5 + 14. In conclusion, the sum is 19.

That's why it's confusing for me because it doesn't add up when 145 = 1 + 4 + 5 = 10. In other words, the sum should be 10, not 19?!?!?????

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Read about recursion - that is the "point" of this function - en.wikipedia.org/wiki/Recursion_(computer_science) – Ron Dadon 10 mins ago
    
Since rest is 14, that means that sumDigits is 14 No, since rest is 14, that means that sumDigits(14) is 5. – torazaburo 9 mins ago
up vote 1 down vote

Let me break it down for you：

sumDigits(145)
5 + sumDigits(14)
5 + 4 + sumDigits(1)
5 + 4 + 1 => 10
share|improve this answer
up vote 0 down vote

The funciton works fine as expected.

Once you get 19 in the end, it runs it again in same function, as 19 is greater than 10. So 1+9 would equal to 10.

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up vote 0 down vote

I think the key here is

Since rest is 14, that means that sumDigits is 14.

If you ran just sumDigit(14), you would sum 1 and 4 = 5. Then 5 is added to the original 5 in 145

When I was thinking this through, I refactored it a bit, which helped me understand what's going on. Here's another way of thinking about it:

function sumDigits(number) {
    var integerDigit = number % 10;

    // If there are digits remaining (i.e. not a single digit number)
    if(number >= 10) {
        // Return the current digit plus sumDigits of remaining digits
        return integerDigit + sumDigits(Math.floor(number / 10));
    }

    // If it is a single digit, just return the number
    return number;
}
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