TypeScript check instance class via user defined type guard

Question

Well, I have implemented

isInstanceOfClass

function, so that it can tell if the instance is instance of given class, now I need to write correct typing for it.

class Parent {

    isInstanceOfClass<T>(arg: T): this is T {

        // already implemented

    }

}



class FooClass extends Parent {

    foo: number;

}



class BarClass extends Parent {

    bar: number;

}

Example:

let foo: Parent;

if(foo.isInstanceOfClass(FooClass)) {

    foo.foo = 1; // TS2339: Property 'foo' does not exist on type 'Parent & typeof FooClass'.

}

Can somebody help me get rid of the error?

For various reasons I can only change the

isInstanceOfClass

method signature, not the example code.

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Answer 1

You're almost there!
It should be:

class Parent {
    isInstanceOfClass<T>(arg: { new(): T }): this is T {
        // already implemented
    }
}

The difference is that what you're passing to isInstanceOfClass is not the instance but the class.
Once you change it to be the class (constructor) then the error goes away.

TypeScript check instance class via user defined type guard

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