```ts
function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Invalid input: expected a string');
  }

  let result = '';
  let count = 1;

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

    if (i + 1 < input.length && char === input[i + 1]) {
      count++;
    } else {
      result += char + String(count);
      count = 1;
    }
  }

  return result;
}

export { runLengthEncode };
```