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Problem 2: Even Fibonacci Numbers

curriculum/challenges/english/blocks/project-euler-problems-1-to-100/5900f36e1000cf542c50fe81.md

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--description--

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:

<div style='text-align: center;'>1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...</div>

By considering the terms in the Fibonacci sequence whose values do not exceed n, find the sum of the even-valued terms.

--hints--

fiboEvenSum(10) should return a number.

js
assert.isNumber(fiboEvenSum(10));

Your function should return an even value.

js
assert.equal(fiboEvenSum(10) % 2 === 0, true);

Your function should sum the even-valued Fibonacci numbers: fiboEvenSum(8) should return 10.

js
assert.strictEqual(fiboEvenSum(8), 10);

fiboEvenSum(10) should return 10.

js
assert.strictEqual(fiboEvenSum(10), 10);

fiboEvenSum(34) should return 44.

js
assert.strictEqual(fiboEvenSum(34), 44);

fiboEvenSum(60) should return 44.

js
assert.strictEqual(fiboEvenSum(60), 44);

fiboEvenSum(1000) should return 798.

js
assert.strictEqual(fiboEvenSum(1000), 798);

fiboEvenSum(100000) should return 60696.

js
assert.strictEqual(fiboEvenSum(100000), 60696);

fiboEvenSum(4000000) should return 4613732.

js
assert.strictEqual(fiboEvenSum(4000000), 4613732);

--seed--

--seed-contents--

js
function fiboEvenSum(n) {

  return true;
}

--solutions--

js
const fiboEvenSum = (number) => {
  if (number <= 1) {
    return 0;
  } else {
    let evenSum = 0,
      prevFibNum = 1,
      fibNum = 2; // According to problem description our Fibonacci series starts with 1, 2
    for (let i = 2; fibNum <= number; i++) {
      if (fibNum % 2 == 0) {
        evenSum += fibNum;
      }
      [prevFibNum, fibNum] = [fibNum, prevFibNum + fibNum];
    }
    return evenSum;
  }
};