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.

Nitzan Tomer · Accepted Answer · 2017-11-29 22:27:02Z

2

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.

answered Nov 29, 2017 at 22:27

Nitzan Tomer

166k51 gold badges330 silver badges307 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

pravdomil Over a year ago

I always forgot the { new(): T } thing!

Collectives™ on Stack Overflow

TypeScript check instance class via user defined type guard

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related