project_euler_2.js

// project euler 2

// Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
// 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
// By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.


// declare initial variables
var a = 0;
var b = 1;
var c = 0;
var sum = 0;

function evenFibSum(num) {
// while the fib number is less than 4 mil
  while (c < num) {
    // create next fib number by adding previous 2 fib numbers
    c = a + b;
    // reassign variables to make new previous 2 fib numbers
    a = b;
    b = c;

    // check if fib num is even
    if (c % 2 === 0) {
      // if even add it to sum variable
      sum += c;
    }
  }
  // return the sum of even fibonacci numbers less than num
  return sum;
}

// log the sum of even fib numbers to the console
console.log(evenFibSum(4000000)); // return 4613732