broken-decode-string.js

const takeNumber = pattern => {
  const [_number] = pattern.match(/^\d+/g) || [];

  return parseInt(_number, 10) || 1;
};

const str = pattern => {
  let count = 1;
  let parts = "";
  let part = "";
  for (let idx = 0; idx < pattern.length; idx++) {
    const char = pattern[idx];
    if (/\d+/.test(char)) {
      if (/\d+/.test(pattern[idx - 1]) === false) {
        count = takeNumber(pattern.slice(idx));
      }
      continue;
    }

    if (char === "]") {
      break;
    } else if (char === "[") {
      let newPart = str(pattern.slice(idx + 1));
      idx += newPart.length + 1;

      part =
        part +
        Array(count)
          .fill(newPart)
          .join("");

      count = takeNumber(pattern.slice(idx));

      parts += part;
      part = "";

      continue;
    } else {
      part += char;
    }
  }

  if (part.length) {
    parts += part;
  }

  return parts;
};

console.log(str("abc"));
console.log(str("3[abc]"));
console.log(str("4[word]2[ab]"));
console.log(str("10[word]"));
console.log(str("2[hey]3[2[b2[c]]"));