function sum_of_even_fibs(lim){ 
var res = 0; 
var prev = 1; 
var n = 1; 
var tmp; 
while (n <= lim){ 
if (n % 2 == 0) { 
res += n; 
} 
tmp = prev; 
prev = n; 
n = n + tmp; 
} 
return res; 
} 
console.log(sum_of_even_fibs(4000000));