Tips for preventing the redeclaration of class method parameter types in child classes in Typescript

Question

Tips for preventing the redeclaration of class method parameter types in child classes in Typescript

I'm encountering an issue with my subclass that extends an abstract superclass in a separate file. The problem arises in the code snippet within implementation.ts, where TS typecheck fails for doStrangeThing() since I have to re-declare the parameter types (string, IComplexOptions). Is there a way to avoid this additional import and repetitive declaration? Any suggestions on how to achieve this?

It seems logical that the compiler should be able to infer the method signature from the superclass, especially if it's abstract, or at the very least consider it as the default if it's missing. Am I overlooking any possible solutions to address this issue (excluding complexities like method overloading or union types)? Is there a method to extract the parameters, provide hints to inherit the signature from the super, or some other approach?

//
// baseClasses.ts
//
import {IComplexOptions} from './foo';

abstract class Animal {
  abstract name: string;
  abstract doStrangeThing(action:string, options:IComplexOptions): void;
}

//
// implementation.ts
//
import {Animal} from './baseClasses';

class Rhino extends Animal {
  name = "Rhino";
  doStrangeThing(action, options) {  // <== TS error here, implicit 'any'

  }
}

javascript typescript

Answer 1

Answer №1

Structural inheritance in Typescript makes it impossible to avoid redeclaring arguments' types.

To work around this limitation, you can utilize the Parameters utility type to extract parameters from another interface. This method is beneficial when using a single object as a function parameter and when argument types may change frequently.

Here's an example:

interface Foo {
  method(args: { arg1: string, arg2: number}): void;
}

class Bar implements Foo {
  method({ arg1, arg2 }: Parameters<Foo['method']>[0]): void {
    // Correctly inferred types for arg1 and arg2
  }
}

If you need to redeclare multiple methods, you can create a custom utility type for a specific class:

interface Foo {
  method(args: { arg1: string, arg2: number}): void;
}

type FooArgs<Method extends keyof Foo> = Parameters<Foo[Method]>[0];

class Bar implements Foo {
  method({ arg1, arg2 }: FooArgs<'method'>): void {
    // Correctly inferred types for arg1 and arg2
  }
}

Answer 2

Structural inheritance in Typescript makes it impossible to avoid redeclaring arguments' types.

To work around this limitation, you can utilize the Parameters utility type to extract parameters from another interface. This method is beneficial when using a single object as a function parameter and when argument types may change frequently.

Here's an example:

interface Foo {
  method(args: { arg1: string, arg2: number}): void;
}

class Bar implements Foo {
  method({ arg1, arg2 }: Parameters<Foo['method']>[0]): void {
    // Correctly inferred types for arg1 and arg2
  }
}

If you need to redeclare multiple methods, you can create a custom utility type for a specific class:

interface Foo {
  method(args: { arg1: string, arg2: number}): void;
}

type FooArgs<Method extends keyof Foo> = Parameters<Foo[Method]>[0];

class Bar implements Foo {
  method({ arg1, arg2 }: FooArgs<'method'>): void {
    // Correctly inferred types for arg1 and arg2
  }
}

Tips for preventing the redeclaration of class method parameter types in child classes in Typescript

Answer №1

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