just out of curiosity: why do you retain the even values and sum them up later while you are only interested in the sum ?
//
let c = 0;
let a = 1;
let b = 2;
let sum = 0;
while (c < 4000000) {
c = a + b;
a = b;
b = c;
if (c % 2 == 0) {
sum += c;
}
}
console.log(sum);
//