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

id, challengeType, isHidden, title, forumTopicId

id	challengeType	isHidden	title	forumTopicId
5900f36e1000cf542c50fe81	5	false	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 whose values do not exceed n, find the sum of the even-valued terms.

Instructions

Tests

tests:
  - text: <code>fiboEvenSum(10)</code> should return a number.
    testString: assert(typeof fiboEvenSum(10) === 'number');
  - text: <code>fiboEvenSum(10)</code> should return 10.
    testString: assert.strictEqual(fiboEvenSum(10), 10);
  - 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);
  - 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 = 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;
  }
}