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

  if (input === "") return "";

  let encoded = "";
  let currentChar = input[0];
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    if (input[i] !== currentChar) {
      encoded += `${currentChar}${count}`;
      currentChar = input[i];
      count = 1;
    } else {
      count++;
    }
  }

  encoded += `${currentChar}${count}`;

  return encoded;
}
```