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The Function

multiplesOf3or5()
multiplesOf3or5(number)

/*
    Returns the sum of all the multiples of 3 or 5 below the number passed in. 
    Additionally, if the number is negative, return 0 (for languages that do have them).

    If the number is a multiple of both 3 and 5, it only counts it once.
*/

multiplesOf3or5(10); 
// 23

multiplesOf3or5(88);
// 1845

Solution by - Need4Swede

How it works

function multiplesOf3or5(number){

    // Create new Array
    let numArr = [];

    // Initalize Sum
    let sum = 0;

    // Count up to the passed-in number  
    for(let i = 1; i < number; i++){
        if(i % 3 == 0 && i % 5 == 0){
            // If the number is divisible by 3 and 5, add it to our new array
            numArr.push(i);
        }
        else if(i % 3 == 0 || i % 5 == 0){
            // If the number is divisible by 3 or 5, add it to our new array
            numArr.push(i)
        }}

    // For every number in our array, add it to the sum
    numArr.forEach(element => {
        sum = element + sum;
    })

    // Return the sum total
        return sum

}

Try it yourself - Codewars Source

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