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

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

  let result = '';
  let currentChar = input[0];
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    const char = input[i];

    if (char !== currentChar) {
      result += currentChar + count;
      currentChar = char;
      count = 1;
    } else {
      count++;
    }
  }

  result += currentChar + count;

  return result;
}
```