Distinguish the components of a standard union in TypeScript

I am looking to incorporate the following feature:

const foo = <S>(newState: S | ((prevState: S) => S)) => {...}

This function should accept either a new state of type S, or a function that generates the new state from the old state.

In my implementation, how can I distinguish between these two options? I have considered using typeof newState === 'function', but I realize that S could also be a function in this case.

Although I am working with TypeScript, I believe the solution may be more related to JavaScript than TypeScript itself.

javascripttypescript

Answer №1

One solution to your issue is to utilize the typeof operator as shown below:

if(typeof newState === 'function') {
    newState();
} else {
    // handle other scenarios
}

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