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

  let result = '';
  let count = 1;

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

  return result;
}
```