```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; ) {
    const char = input[i];
    let count = 1;

    while (i + 1 < input.length && input[i] === input[i + 1]) {
      count++;
      i++;
    }

    encoded += `${char}${count}`;
    i++;
  }

  return encoded;
}
```