freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-2-even-fibonacci-numbers.english.md

---
id: 5900f36e1000cf542c50fe81
challengeType: 5
title: 'Problem 2: Even Fibonacci Numbers'
forumTopicId: 301838
---

## Description

<section id='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 <code>n</code>, find the sum of the even-valued terms.

</section>

## Instructions

<section id='instructions'>

</section>

## Tests

<section id='tests'>

```yml
tests:
  - text: <code>fiboEvenSum(10)</code> should return a number.
    testString: assert(typeof fiboEvenSum(10) === 'number');
  - text: Your function should return an <code>even</code> value.
    testString: assert.equal(fiboEvenSum(10) % 2 === 0, true);
  - text: 'Your function should sum the even-valued Fibonacci numbers: <code>fiboEvenSum(8)</code> should return 10.'
    testString: assert.strictEqual(fiboEvenSum(8), 10);
  - text: <code>fiboEvenSum(10)</code> should return 10.
    testString: assert.strictEqual(fiboEvenSum(10), 10);
  - text: <code>fiboEvenSum(34)</code> should return 44.
    testString: assert.strictEqual(fiboEvenSum(34), 44);
  - text: <code>fiboEvenSum(60)</code> should return 44.
    testString: assert.strictEqual(fiboEvenSum(60), 44);
  - text: <code>fiboEvenSum(1000)</code> should return 798.
    testString: assert.strictEqual(fiboEvenSum(1000), 798);
  - text: <code>fiboEvenSum(100000)</code> should return 60696.
    testString: assert.strictEqual(fiboEvenSum(100000), 60696);
  - text: <code>fiboEvenSum(4000000)</code> should return 4613732.
    testString: assert.strictEqual(fiboEvenSum(4000000), 4613732);
```

</section>

## Challenge Seed

<section id='challengeSeed'>

<div id='js-seed'>

```js
function fiboEvenSum(n) {

  return true;
}
```

</div>

</section>

## Solution

<section id='solution'>

```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;
  }
};
```

</section>