reducer_challenges.js

// Challenge one: Add all the numbers in array
array.reduce((a, e) => a + e);

// Challenge two: Create a string with all the numbers stitched in one string
array.join('')
array.reduce((a, e) => `${a}${e}`)
array.reduce((a, e) => '' + a + e);
array.reduce((a, e) => a + e.toString(), '');
array.reduce((a, e) => a + e, '');
array.reduce((a, e) => a.toString() + e.toString());

// Challenge three: Find the maximum/minimum number of all numbers
Math.min(...array);
Math.min.apply(null, array)
array.reduce((e1, e2) => (e1 < e2 ? e1 : e2));

Math.max(...array);
array.reduce((e1, e2) => (e1 < e2 ? e1 : e2));

// Challenge: Counting the zeros
array.filter(el => el === 0).length

// Challenge four: Find the median number of all numbers
[...array]
  .sort((a, b) => a - b)
  .reduce((a, e, i, arr) => {
    if (Math.abs(i - (arr.length - 1) / 2) < 1)
      return a + e / (1 << (1 - (array.length & 1)));
    else return a;
  }, 0);

// IF YOU DO NOT UNDERSTAND THE ABOVE TRY THIS BABY-STEP
[...array].sort((a, b) => a - b).reduce((a, e, i, arr) => {
  if (Math.abs(i - (arr.length - 1) / 2) < 1)
    console.log(`${e} is at the center of array at i=${i}`);
  return 0;
}, 0);
console.log(
`the elements will have to be divided by ${1 << (1 - (array.length & 1))}`
);

// Arkadii's solution, genius, sorts and counts in one reduce sentence
array.reduce( 
  (res, el) => [
    // Incrementally sorting the array by spreading the values under and above el
    [...res[0].filter(n => n <= el), el, ...res[0].filter(n => n > el)],
    // Counting the elements in the array 
    ++res[1], 
    // Adds the two middle elements and divides by 2
    (res[0][ Math.floor( (res[1] - 1) / 2 ) ] + res[0][ Math.ceil( (res[1]- 1) / 2 ) ]) / 2 ] , [ [] , 0, 0 ] ) [2]

// Challenge five: Find the longest streak of zeros found in the array
Math.max(
  ...array.reduce(
    (a, e, i, arr) => {
      const p = i > 0 ? arr[i - 1] : null;
      if (e === 0)
        if (p !== 0) return [Math.max(...a), 1];
        else return [a[0], a[1] + 1];
      else return a;
    },
    [0, 0]
  )
);

// GENIUS: ARKADII
array.reduce(
  (res, e) =>
    e === 0 ? [++res[0], res[1]] : res[0] > res[1] ? [0, res[0]] : [0, res[1]],
  [0, 0]
)[1];

// Challenge six: Extract the length of all the streaks of zeros found in the array
array.reduce((a, e, i, arr) => {
  const p = i > 0 ? arr[i - 1] : null;
  if (e === 0)
    if (p !== 0) {
      a.push(1);
      return a;
    } else {
      a[a.length - 1]++;
      return a;
    }
  else return a;
}, []);

// GENIUS ARKADII
array.reduce(
  (res, e, i, arr) =>
    arr[arr.length - 1 - i] === 0
      ? [++res[0], res[1]]
      : res[0]
      ? [0, [...res[1], res[0]]]
      : [0, [...res[1]]],
  [0, []]
)[1];

// RIGHT VERSION
array.reduceRight(
  (res, e, i, arr) =>
    arr[arr.length - 1 - i] === 0
      ? [++res[0], res[1]]
      : res[0]
      ? [0, [...res[1], res[0]]]
      : [0, [...res[1]]],
  [0, []]
)[1];

// EQUIVALENT RIGHT VERSION?
array.reduce(
  (res, e, i, arr) =>
    arr[arr.length - 1 - i] === 0
      ? [++res[0], res[1]]
      : res[0]
      ? [0, [...res[1], res[0]]]
      : [0, [...res[1]]],
  [0, []]
)[1];