How to convert unknown type to string in TypeScript?

Question

I'm dealing with an interface from an external piece of code. Boiled down to the minimum it looks like this:

interface Input {
    details?: unknown;
}

Now I need to map this object into different object with type:

interface Output {
    message?: string;
}

So unknown must be cast to a string. My initial approach was simply calling input.details.toString(), but toString is not available because unknown can also be null or undefined.

How do I transform Input to Output?

Answer 1

To cast unknown to string you can use a typeof type guard. The type of the ternary expression below will be inferred as string | undefined:

const output: Output = {
    message: typeof input.details === 'string'
        ? input.details
        : undefined,
};