const SQ5 = 5 ** .5; // Square root of 5 
const PHI = (1 + SQ5) / 2; // definition of PHI

//theoretically it should take O(1) constant amount of time as long
// arithmetic calculations are considered to be in constant amount of time.

function EvenFibonacci(limit) {
    const highestIndex = Math.floor(Math.log(limit * SQ5) / Math.log(PHI));
    const n = Math.floor(highestIndex / 3);
    return ((PHI ** (3 * n + 3) - 1) / (PHI ** 3 - 1)
        - ((1 - PHI) ** (3 * n + 3) - 1) / ((1 - PHI) ** 3 - 1)) / SQ5;
}

console.log(EvenFibonacci(4e6));
// Sum of Even Fibonnaci upto 4 Million