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

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

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

  encoded += `${currentChar}${count}`; // Add the last run

  return encoded;
}
```