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

  let encoded = '';
  let i = 0;

  while (i < input.length) {
    const char = input[i];
    let count = 1;
    while (i + 1 < input.length && input[i + 1] === char) {
      i++;
      count++;
    }
    encoded += `${char}${count}`;
    i++;
  }

  return encoded;
}
```