I wanted to practice functional programming (fp) without using any library but using vanilla JS only. So I took a problem from Project Euler:

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

My solution in FP:

(function () {
  'use strict';

  const INPUT =    '7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450';

  const parser = (maxProduct, number) => {
    const SIZE = 13;
    const multiply = (acc, val) => acc * val;
    const product = number
      .slice(0, SIZE)
    const latestMaxProduct = product > maxProduct ? product : maxProduct;

    return number.length < SIZE ?
      latestMaxProduct :
      parser(latestMaxProduct, number.slice(1));
  const solution = parser(0, INPUT);

  console.log("solution ", solution);

Is there a better way to write it in FP (without any libraries and with vanilla JS only)? Also any improvement suggestions are welcomed!

  • \$\begingroup\$ I have nothing for the FP aspect of it, but the style is inconsistent. Some const are, according to standards, all uppercase, but the rest are not. \$\endgroup\$ Jan 4, 2018 at 19:59
  • \$\begingroup\$ SIZE and INPUT are capital letters because they stay the same at every iteration. Others are functions. And others are indeed constant-variables, but different ones - newly created at every recursion. This is the distinction I wanted to make between const with capital letters and const with standard letters. @Carles Alcolea \$\endgroup\$ Jan 4, 2018 at 20:16
  • \$\begingroup\$ Instead of (function() {})()you could use a simple block {}. \$\endgroup\$
    – le_m
    Jan 4, 2018 at 22:04
  • \$\begingroup\$ The products of 13 digits are still within the range where you can perform integer division without having to worry about precision errors. So a less complex solution would probably find those products by iterating through all digits only once, multiplying with the next digit and dividing by the digit seen 13 iterations ago. You could squeeze this into a reduce accumulator, but I'd probably write a generator function for that and a function max(iterable). \$\endgroup\$
    – le_m
    Jan 4, 2018 at 22:29
  • 2
    \$\begingroup\$ For those downvoting this question: Please provide an explanation so I don't repeat the mistake twice. \$\endgroup\$ Jan 5, 2018 at 11:29

1 Answer 1


Some notes about your code:

  • (function () {. Is this old-style wrapping really needed? Isn't this a node.js script?
  • You parsed INPUT manually from the question. You should probably leave the input as similar as possible to how it's stated and do the parsing programatically.
  • const multiply = (acc, val) => acc * val;: The variable names reveal how the function is used; instead, keep it generic: const multiply = (x, y) => x * y;. Consider also creating the abstraction product, the product of all numbers in an array.
  • parser(latestMaxProduct, number.slice(1)); You should use recursive calls as a last resort, only when higher-level abstractions (map, filter, reduce, ...) are not enough.
  • I'd separate generic functions from the specific code used in the problem. Re-using generic abstractions is one the fundamental principles of FP.

I'd write:

const range = (start, end) => Array.from(new Array(end - start), (x, i) => i + start);
const groupsOf = (xs, n) => range(0, xs.length - n + 1).map(i =>  xs.slice(i, i + n));
const product = xs => xs.reduce((acc, x) => acc * x, 1);
const maximum = xs => Math.max(...xs);

const euler8 = () => {
  const unparsedDigits = `
  const groupSize = 13;

  const digits = unparsedDigits.replace(/[^\d]/g, "").split("").map(c => parseInt(c));
  return maximum(groupsOf(digits, groupSize).map(product));

  • 2
    \$\begingroup\$ May downvoters comment so the answer can be improved? \$\endgroup\$
    – tokland
    Jan 4, 2018 at 23:08

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