Project Euler solutions in JS

euler.js

// Some of these answers will only run in Firefox (browsers which support TCO)

/**
 * Problem 1
 * If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
 * Find the sum of all the multiples of 3 or 5 below 1000.
 */
const problem1 = () => {
  return Array(1000)
    .fill(null)
    .map((_, index) => index + 1)
    .filter(num => num % 3 === 0 || num % 5 === 0)
    .reduce((sum = 0, num) => sum + num);
};

/**
 * Problem 2
 * 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.
 */
const problem2 = () => {
  const fibs = function*() {
    const fib = (n, a = 1, b = 2) => {
      if (n === 1) {
        return a;
      } else if (n === 2) {
        return b;
      } else {
        return fib(n - 1, b, a + b);
      }
    };

    let n = 1;
    while (true) {
      yield fib(n++);
    }
  };

  const fibsUnder4000000 = () => {
    const arr = [];
    for (let value of fibs()) {
      if (value > 4000000) break;
      arr.push(value);
    }
    return arr;
  };

  return fibsUnder4000000()
    .filter(num => num % 2 === 0)
    .reduce((sum = 0, fib) => sum + fib);
};

/**
 * Problem 3
 * The prime factors of 13195 are 5, 7, 13 and 29.
 * What is the largest prime factor of the number 600851475143 ?
 */
const problem3 = () => {
  return (() => {
    const isMultiple = (n, div) => {
      return n % div === 0;
    };

    const primeFactors = (n, candidate = 2, acc = []) => {
      if (n <= 1) {
        return [...acc].reverse();
      } else if (isMultiple(n, candidate)) {
        return primeFactors(n / candidate, candidate, [...acc, candidate]);
      } else {
        return primeFactors(n, candidate + 1, [...acc]);
      }
    };

    return Math.max(...primeFactors(600851475143));
  })();
};