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

  let encoded = "";
  for (let i = 0; i < input.length; i++) {
    const char = input[i];
    let count = 1;

    // Count consecutive characters
    while (i + count < input.length && input[i + count] === char) {
      count++;
    }

    encoded += char + count.toString();
    i += count - 1;
  }

  return encoded;
}
```