Группировка одинаковых значений из массива JavaScript

let array = ['a', 'a', 'a', 'b', 'b', 'c'];

let result = Object.entries(
    array.reduce((r, c) => (r[c] = (r[c] || 0) + 1, r) , {})
).map(([k, v]) => v == 1 ? k : v + k);

console.info(result);

let array = ['a', 'a', 'a', 'b', 'b', 'c'];

let result = (((arr, map) => {
    arr.forEach(s => map.set(s, (map.get(s) || 0) + 1));
    return [...map.entries()].map(([k, v]) => v == 1 ? k : v + k);
})(array, new Map()));

console.info(result);

let array = ['a', 'a', 'a', 'b', 'b', 'c'];

let countObj = array.reduce((acc, val) => (acc[val] = acc[val] ? acc[val] + 1 : 1, acc), {});

console.info(countObj);

let countArr = Object.keys(countObj).map(key => '' + countObj[key] + key);

console.info(countArr);

const arr = ['a', 'a', 'a', 'b', 'b', 'c'];

// `reduce` over the arr
const obj = arr.reduce((acc, c) => {

  // If the object (acc) we passed in doesn't have
  // a key assigned to the letter in the current
  // iteration (c) add it, set it to 0, then
  // add one, otherwise, if there is a key, add one
  acc[c] = (acc[c] || 0) + 1;

  // Return the object for the next iteration
  return acc;

// Pass in an initial object 
}, {});

console.info(obj);

// Grab the value of `a` property
console.info(obj.a);

let array = ['a', 'a', 'a', 'b', 'b', 'c'],
    result = array.reduce(
        (r, c, i, a) => r.concat(c === a[i - 1]
            ? ((+r.pop().slice(0, -1) || 1) + 1) + c
            : c),
        []
    );

console.info(result);