Creating a TypeScript function that automatically infers the type of the returned function using generics

Suppose I want to execute the generateFunction() method which will yield the following function:

// The returned function
const suppliedFunction = <T>(args: T) => {
    return true;
}; // The returned function

// This is how it can be used
suppliedFunction({ prop: "value" }); 

I have chosen to construct the generatorFunction like this:

// Function generator, returning the supplied function from the parameter
// The supplied function should return a boolean
function generateFunction(fn: <T>(args: T) => boolean) {
    return fn;
}

// However, I encountered an error with this code
// Property 'prop' does not exist on type 'unknown'
const resultFunction = generateFunction(({ prop }) => {
    return true;
});

My intention is to utilize resultFunction with type inference derived from the generated function (instead of explicitly specifying the type)

// This should result in an error
// Argument of type 'string' is not assignable to parameter of type '{ prop: any; }'
resultFunction('text');

// This should work fine
resultFunction({ prop: "winter" });

How can we correctly implement these generics for my scenario? Thank you in advance.

Answer №1

Finally, after days of searching and problem-solving, I discovered exactly what I needed. Huge thanks to @jcalz for providing the solution in this insightful post reply. By incorporating custom ArgumentTypes and ReturnTypes based on generics outlined in the answer, I was able to overcome limitations with TypeScript's Parameters<> and ReturnType<> utilities in my project.

type Serializables =
    | Function
    | PropertyDescriptor
    | Document
    | Omit<RegExp, "lastIndex">
    | never;

type SerializableParam<Serializable> = Serializable extends Serializables
    ? unknown
    : Serializable;

// More code...

export default function generateFunction<F>(
    fn: F & SupplierFunction<F, ArgumentTypes<F & Function>[0]>
) {
    return async (payload: ArgumentTypes<F & Function>[0]) => {
        const result: SerializableReturnType<ReturnType<F & Function>> =
            await new Promise((resolve) => {
                resolve(fn(payload));
            });
        return await result;
    };
}

This method works seamlessly, allowing dynamic functionality and supporting manipulation of overloading functions for type safety. To exclude passing document object model values as parameters and returning specific types from the supplied function in the generator, simply add them to the Serializables type for restriction purposes.

See it in action here

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