yo.js

function parse(input) {
  input = input.split('');
  const stack = [];
  const isChar = (char) => /[a-z]/.test(char);
  const isNum = (char) => /[0-9]/.test(char);

  for (const char of input) {
    if (char === '[') {
      stack.push(char);
    } else if (isChar(char)) {
      stack.push(char);
    } else if (isNum(char)) {
      stack.push(char);
    } else if (char === ']') {
      const slice = [];

      while (stack[stack.length - 1] !== '[') {
        slice.unshift(stack.pop());
      }

      stack.pop();

      if (isNum(stack[stack.length - 1])) {
        const num = stack.pop();
        for (let i = 0; i < num; i++) {
          stack.push(...slice);
        }
      } else {
        stack.push(...slice);
      }
    }
  }

  return stack.join('');
}