How in god's name does this work?

The_Aptly_Named_I_Dont_Know_How_This_Works.js

// How in gods name did this work? I don't understand what I even did. It started off making sense and 25 console logs and muckeries later
// it has come to this. Literally wtf. 

function sumPrimes(num){

    var i = 2, nums = [], primes = [];

    while(i <= num){
        nums.push(i);
        i++;
    }

    for(i in nums){
        primes.push(i_dont_know_how_this_works(nums[i], nums));
    }


    function flatten(a,b){
        return a.concat(b);
    }

    function sum(a,b){
        return a+b
    }


    function i_dont_know_how_this_works(num, arr){
        var i = 0;
        var primes = [];

        for(i; i < arr.length; i++){
            if(num % arr[i] === 0){
                if(arr[i] !== 1 || arr[i] !== num){
                    primes.push(arr[i]);
                }
            }
        }

        if(primes.length === 1){
            return primes;
        }

        else{
            return false;
        }

    }


    return primes.filter(function(nums){return nums}).reduce(flatten).reduce(sum);

}