```ts
export function runLengthEncode(input: string): string {
  if (typeof input !== "string") {
    throw new Error("Argument must be a string");
  }
  if (input.length === 0) {
    return "";
  }

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

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

  return encoded;
}
```