Typescript Generic Class Constructor Argument with static methods

Question

So the basic problem I have is this.

I have a base class from which all other classes are derived. It has a static method. Something like this:

class BaseClass {
  static blah() : string {
    return 'base';
  }
}

class OtherClass extends BaseClass {
  static blah() : string {
    return 'other';
  }
}

class DoesntExtend extends SomeOtherClass {

}

Then I have a function which I want to pass in the Class Constructor and call the static method, but I'm having trouble with the method signature.

I want to use it something like this:

console.log(funcCaller(BaseClass)) -> outputs 'base'
console.log(funcCaller(OtherClass)) -> outputs 'other'
console.log(funcCaller(DoesntExtend)) -> won't compile, since DoesntExtend won't have the static method

So I thought to do something like:

type Constructor<T extends BaseClass > = new(...args: any[]) => T;

function funcCaller<T extends BaseClass>(c : Constructor<T>) : string {
  return c.blah();
}

but tsc complains that

Property 'blah' does not exist on type 'Constructor'

any ideas?

Nishant · Accepted Answer · 2021-01-28 04:21:03Z

1

The funcCaller function needs to accept typeof the class to access the static properties. This is because static properties exist on the type while the signature you have used accepts an instance of that type. As the docs says, "To refer to the type that the value f has, we use typeof".

TS Playground

class BaseClass {
  static blah() : string {
    return 'base';
  }
}

class OtherClass extends BaseClass {
  static blah() : string {
    return 'other';
  }
}

class DoesntOverride extends BaseClass{

}

class SomeOtherClass {
}

class DoesntExtend extends SomeOtherClass {

}

function funcCaller(c : typeof BaseClass) : string {
    return c.blah();
}

// base
console.log(funcCaller(BaseClass));

// other
console.log(funcCaller(OtherClass));

// base
console.log(funcCaller(DoesntOverride));

// Shows error
console.log(funcCaller(DoesntExtend));

edited Jan 28, 2021 at 4:21

answered Jan 28, 2021 at 3:53

Nishant

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2 Comments

Richard Howell-Peak Over a year ago

Wow thanks, didn't even know you could use typeof like that,

Jimbly Over a year ago

Great, this code works and solves a similar problem for me! The description is, however, incorrect - the code as written in the OP does not accept an instance of the type (e.g. funcCaller(new OtherClass()) will generate an error), but only constructors that derive from BaseClass, which is almost exactly the same set of things that typeof BaseClass accepts (except, the latter, I guess, somehow pulls along the whole class's type, not just the constructor's signature itself, not that there's actually any difference in the underlying JS ^_^).

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