008.js

/**
 * Largest Product in A Series
 * Computes the largest product of thirteen adjacent digits
 *
 * Ashen Gunaratne
 * mail@ashenm.ml
 *
 */

/**
 * @constant
 * @type {String}
 */
const sequence = '73167176531330624919225119674426574742355349194934'
  + '96983520312774506326239578318016984801869478851843'
  + '85861560789112949495459501737958331952853208805511'
  + '12540698747158523863050715693290963295227443043557'
  + '66896648950445244523161731856403098711121722383113'
  + '62229893423380308135336276614282806444486645238749'
  + '30358907296290491560440772390713810515859307960866'
  + '70172427121883998797908792274921901699720888093776'
  + '65727333001053367881220235421809751254540594752243'
  + '52584907711670556013604839586446706324415722155397'
  + '53697817977846174064955149290862569321978468622482'
  + '83972241375657056057490261407972968652414535100474'
  + '82166370484403199890008895243450658541227588666881'
  + '16427171479924442928230863465674813919123162824586'
  + '17866458359124566529476545682848912883142607690042'
  + '24219022671055626321111109370544217506941658960408'
  + '07198403850962455444362981230987879927244284909188'
  + '84580156166097919133875499200524063689912560717606'
  + '05886116467109405077541002256983155200055935729725'
  + '71636269561882670428252483600823257530420752963450'
  ;

/**
 * @function
 * @summary Computes the i-th largest product of n adjacent digits
 * @param {Number} i
 * @param {Number} n
 * @param {Number|String} s
 * @description Returns the i-th largest product composing of n adjacent digits in the parameterised sequence s
 */
const largest = function computeLargestProduct(i, n, s) {

  let products = [];

  const ceil = s.toString().length - n;

  for (let index = 0; index <= ceil; index++) {

    let product = 1;

    for (let slice = 0; slice < n; slice++) {
      product *= +s[index + slice];
    }

    products.push(product);

  }

  products.sort((a, b) => a > b ? -1 : 1);

  return products[i - 1];

};

// tests
console.assert(largest(1, 4, '112211') === 4, `expected 4 got ${largest(1, 4, '112211')}`);
console.assert(largest(1, 4, sequence) === 5832, `expected 5832 got ${largest(1, 4, sequence)}`);

// answer
console.log(largest(1, 13, sequence));