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

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

    let encoded = '';
    let count = 1;
    for (let i = 1; i <= input.length; i++) {
        if (i < input.length && input[i] === input[i - 1]) {
            count++;
        } else {
            encoded += input[i - 1] + count;
            count = 1;
        }
    }
    return encoded;
}
```