nCoding.js

function letterFrequency(str) {
  const uniques = {};
  for (let char of str) {
    uniques[char] = uniques[char] ? ++uniques[char] : 1;
  }

  return uniques;
}

function letterFrequencyTemplate(uniques) {
  return Object.entries(uniques)
    .map(([char, count]) => `${char}${count}`)
    .join("");
}

function compress(str) {
  let compressed = "";

  let lastChar;
  let count;
  for (let char of str) {
    if (char === lastChar) {
      count++;
      continue;
    }

    if (lastChar) {
      compressed += lastChar;
      if (count > 1) {
        compressed += `${count}`;
      }
    }

    lastChar = char;
    count = 1;
  }

  if (lastChar) {
    compressed += lastChar;
    if (count > 1) {
      compressed += `${count}`;
    }
  }

  return compressed;
}

function compress2(str) {
  let compressed = "";

  function append(currentChar, count) {
    compressed += currentChar;
    if (count > 1) {
      compressed += `${count}`;
    }
  }

  let lastChar;
  let count;

  for (let char of str) {
    if (char === lastChar) {
      count++;
      continue;
    }

    if (lastChar) append(lastChar, count);

    lastChar = char;
    count = 1;
  }

  if (lastChar) append(lastChar, count);

  return compressed;
}

function compress3(str, separator = "") {
  let compressed = "";

  function append(currentChar, count) {
    if (compressed.length) {
      compressed += separator;
    }

    compressed += currentChar;
    if (count > 1) {
      compressed += `${count}`;
    }
  }

  let lastChar;
  let count;

  for (let char of str) {
    if (char === lastChar) {
      count++;
      continue;
    }

    if (lastChar) append(lastChar, count);

    lastChar = char;
    count = 1;
  }

  if (lastChar) append(lastChar, count);

  return compressed;
}

function* reverse(str) {
  for (let idx = str.length - 1; idx >= 0; idx--) {
    yield str[idx];
  }
}

function decompress(str) {
  const NUMBER_REGEX = /\d/;

  let decompressed = "";

  function append(currentChar, _count) {
    const count = Number(_count.length ? _count : 1);

    let chunk = currentChar;
    if (count > 1) {
      chunk = new Array(count).fill(currentChar).join("");
    }

    decompressed = `${chunk}${decompressed}`;
  }

  let count = "";
  for (let char of reverse(str)) {
    if (NUMBER_REGEX.test(char)) {
      count = `${char}${count}`;
      continue;
    }

    append(char, count);

    count = "";
  }

  return decompressed;
}

console.log(letterFrequencyTemplate(letterFrequency("aabbbcccccdd")));
console.log(compress("aabbbcccccdd"));
console.log(compress2("aabbbcccccdd"));
console.log(compress3("xyyzzz", ","));

console.assert(
  compress3("aabbbcccccdd") === "a2b3c5d2",
  "Could not compress correctly!"
);

// NOTE:
// Without a separator it is not possible to decompress strings with numbers in them
// because if you see something like 'a2b34', there could be 34 b characters at the
// end or 3 b characters followed by a 4 character.
// However, if there is a separator then these are distinct (e.g. 'a2,b34' and 'a2,b3,4').
console.log(decompress("a2b3c5d2"));
console.log(decompress("x7y2z14"));
console.assert(
  decompress("a2b3c5d2") === "aabbbcccccdd",
  "Failed to decompress!"
);

See also: https://gist.github.com/sebinsua/763998bee8efb3d7e1d04ec53dea5349