freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-2-even-fibonacci-numbers.english.md

id, challengeType, title, forumTopicId

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

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:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence that do not exceed the nth term, find the sum of the even-valued terms.

Instructions

Tests

tests:
  - text: <code>fiboEvenSum(10)</code> should return 44.
    testString: assert.strictEqual(fiboEvenSum(10), 44);
  - text: <code>fiboEvenSum(18)</code> should return 3382.
    testString: assert.strictEqual(fiboEvenSum(18), 3382);
  - text: <code>fiboEvenSum(23)</code> should return 60696.
    testString: assert.strictEqual(fiboEvenSum(23), 60696);
  - text: <code>fiboEvenSum(43)</code> should return 350704366.
    testString: assert.strictEqual(fiboEvenSum(43), 350704366);
  - text: Your function should return an <code>even</code> value.
    testString: assert.equal(fiboEvenSum(10) % 2 === 0, true);

Challenge Seed

function fiboEvenSum(n) {
  // You can do it!
  return true;
}

fiboEvenSum(10);

Solution

const fiboEvenSum = (number) => {
  if (number <= 1) {
    return 0;
  } else {
      let evenSum = 2, first = 1, second = 2, fibNum; // According to problem description our Fibonacci series starts with 1, 2
      for (let i = 3; i <= number; i++) {
        fibNum = first + second;
        first = second;
        second = fibNum;
        if (fibNum % 2 == 0) {
          evenSum += fibNum;
        }
      }
      return evenSum;
  }
}