CODEWITHSUNDEEP
typescripttypescript2.0
POV
POV

Reputation: 12005

Override abstract method TypeScript?

I have abstract method:

abstract setProp<T, K extends keyof T>(value: keyof T, key: K);

I tried to override this in class heir:

public setProp<IParentProfile, K extends keyof IParentProfile>(value: keyof IParentProfile, key: K) {
    this.model[key] = value;
  }

But interpretation says me an error:

Incompatible override method from abstract class, where did I make mistake?

Also I tried this way:

abstract setProp<T, K extends keyof T>(model: T, value: keyof T, key: K): T;

And using:

public setProp<IParentProfile, K extends keyof IParentProfile>(model: IParentProfile, value: keyof IParentProfile, key: K) {
    return model[key] = value;
  }

Can you see why this does not work please

Upvotes: 3

Views: 1834

Answers (1)

jcalz
jcalz

Reputation: 327819

As I said in the comments, it looks like a compiler bug. (This is only addressing the inability to override your generic method, not the code you added in the link later.)

The following is currently an error in TypeScript 2.9 and below:

class A {
  m<T, U extends T>(t: T, u: U) { }
}

class B extends A {
  m<T, U extends T>(t: T, u: U) { }
//^--error, T is not assignable to T
}

It seems that you can't override a generic method if its generic constraints use type parameters.

I filed Microsoft/TypeScript#25373 and it has (as of 2018-07-02) been classified as a bug, which is slated to be addressed in TypeScript 3.1.

Until then, I guess you need to use workarounds. Actually, the only workaround I can find that lets you override abstract generic methods using type-parameter constraints is the sledgehammer of workarounds, the @tsignore comment:

class A {
  m<T, U extends T>(t: T, u: U) { }
}

class B extends A {
  // @ts-ignore
  m<T, U extends T>(t: T, u: U) { } // no error
}

I don't normally recommend using @ts-ignore, because all it does is suppress the error output; it doesn't magically fix anything. But assuming the compiler bug gets fixed in the near future, it's a reasonable temporary fix.

Hope that helps; good luck!

Upvotes: 4

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