```typescript
export function runLengthEncode(input: string): string {
  if (typeof input !== 'string') {
    throw new Error('Input must be a string');
  }

  if (input === '') {
    return '';
  }

  let result = [];
  let count = 1;

  for (let i = 0; i < input.length; i++) {
    if (i + 1 < input.length && input[i] === input[i + 1]) {
      count++;
    } else {
      result.push(input[i], count.toString());
      count = 1;
    }
  }

  return result.join('');
}
```