JSFiddle - React, Tailwind, and code Playground

JavaScript

function multiples(n){
    var sum = 0;
    for(var i = 0; i < n; i++){
        if( !(i % 3) || !(i % 5) ){
            sum += i;
        }
    }
    return sum;
}
function evenFib(){
    var nums = [1,2];
    var lastNum = 2;
    var sum = 2;
    var i = 1;
    while(nums[i] + nums[i-1] < 4000000){
        lastNum = nums[i] + nums[i-1];
        if( !(lastNum % 2)){
            sum += lastNum;
        }
        nums.push(lastNum);
        i++;
    }
    return sum;
}
console.log(multiples(1000));
console.log(evenFib());