002.js

/*
Source: https://projecteuler.net/problem=2

Problem:

"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."



My Algorithm in words:

1. Make an array of fibonacci numbers with numbers from 1 to 4 million
2. Use a for loop on that array to extract the even numbers
3. Get the sum of the array with the even numbers
*/

function fibonacciUntil4Mil() {
  let term1 = 1;
  let term2 = 1;

  let fibonacci = [1, 1]; 

  let nextTerm = 2;

  for (let i = 0; i <= 100; i++) { //The number 100 is kinda arbitrary
    let nextTerm = term1 + term2; //next term is the sum of the last two numbers
    term1 = term2; //shift term1 and term2 forward
    term2 = nextTerm; //shift term1 and term2 forward
    if (nextTerm < 4000000) { //push to the fibonacci numbers array until the terms reach 4,000,000
       fibonacci.push(nextTerm);
    }
  }

  return fibonacci; //return the array
}

function solution() {
  let fibonacci = fibonacciUntil4Mil(); //get the fibonacci numbers array
  let evenFibonacciNumbers = [];

  for (let number of fibonacci) {
    if (number % 2 === 0) { //check if the number is even by checking if the remainder when divided by 2 is 0
      evenFibonacciNumbers.push(number); //if so, push it to the even fibonacci numbers array
    }
  }

  const add = (a, b) => a + b; //arrow function to add numbers
  return(evenFibonacciNumbers.reduce(add)); //add the numbers in the evenFibonacciNumbers array and return it
}

console.log(solution()); //outputs the answer!