euler_problem_2.js

/*
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.
O,E,O,O,E,O,O,E,O,O,
A number in the Fibonacci sequence can only be even if its 2 preceding terms are either both even or both odd, as it turns out, every 3rd term is even.
As it also turns out, there isn't really a whole lot I can do with this information.
*/
const euler_2 = n => {
  let result = 0,
      i = 1;
  while (fibonacci(i) < n) {
    i++;
    let f = fibonacci(i);
    if(!(f & 1)) {
      result += f;
    }
  }
  return result;
}

// Function to work out the fibonacci numbers. Done as an array because it is much much faster than using a recursive function.
const fibonacci = n => {
  if (!Number.isInteger(n) || n < 1) {
    return undefined;
  }
  if (n === 1 || n === 2) {
    return n;
  }
  let fibarray = [1,2];
  for (let i = 2; i < n; i++) {
    fibarray[i] = fibarray[i-1] + fibarray[i-2];
  }
  return fibarray[n-1];
}