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

  const result: string[] = [];
  for (let i = 0; i < input.length;) {
    let count = 1;
    while (i + 1 < input.length && input[i] === input[i + 1]) {
      count++;
      i++;
    }
    result.push(input[i] + count.toString());
    i++;
  }

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