See: https://codesandbox.io/s/practical-mclaren-c79zt8?file=/src/index.js:0-1360

minimal-rle.js

function encodeRLE(str) {
  let buffer = "";

  let currentChar;
  let count = 0;
  for (let char of str) {
    if (char !== currentChar) {
      if (currentChar && count > 0) {
        buffer += `${currentChar}${count > 1 ? String(count) : ""}`;
      }
      currentChar = char;
      count = 1;
    } else {
      currentChar = char;
      count++;
    }
  }

  if (currentChar && count > 0) {
    buffer += `${currentChar}${count > 1 ? String(count) : ""}`;
  }

  return buffer;
}

const isNumber = (char) =>
  char === "0" ||
  char === "1" ||
  char === "2" ||
  char === "3" ||
  char === "4" ||
  char === "5" ||
  char === "6" ||
  char === "7" ||
  char === "8" ||
  char === "9";

function decodeRLE(str) {
  let buffer = "";

  for (let index = 0; index < str.length; index++) {
    const char = str[index];

    let numericIndex = index;
    let number = "";
    while (isNumber(str[numericIndex + 1])) {
      numericIndex++;
      number += str[numericIndex];
    }

    buffer +=
      number.length > 0
        ? Array(parseInt(number, 10)).fill(char).join("")
        : char;

    // The if block isn't required but it makes the intent clearer.
    if (index !== numericIndex) {
      index = numericIndex;
    }
  }

  return buffer;
}

console.log(encodeRLE("nooooooooothereisnootherway"));
console.log(decodeRLE("no9thereisno2therway"));

See also: https://gist.github.com/sebinsua/97a8bca51188903d3199c1dad5694c96