Enhance your TypeScript classes with decorators that can add new methods to your class

Question

Enhance your TypeScript classes with decorators that can add new methods to your class

Can you explain property definition using TypeScript and decorators?

Let's take a look at this class decorator:

function Entity<TFunction extends Function>(target: TFunction): TFunction {
    Object.defineProperty(target.prototype, 'test', {
        value: function() {
            console.log('test call');
            return 'test result';
        }
    });
    return target;
}

Here is how it's used:

@Entity
class Project {
    //
}

let project = new Project();
console.log(project.test());

After running this code, you'll see the following output in the console:

test call            entity.ts:5
test result          entity.ts:18

Although the code works correctly, TypeScript shows an error:

entity.ts(18,21): error TS2339: Property 'test' does not exist on type 'Project'.

Do you know how to resolve this issue?

typescript annotations decorator

Answer 1

Answer №1

As far as my current knowledge goes, this capability is currently not feasible. There is an ongoing discussion regarding this matter which can be found here: discussion.

Therefore, one option is to refrain from using decorators for extending classes and consider utilizing interfaces with declaration merging for expanding type declarations. Another alternative would be to use type assertion (even though it may compromise type checks): (<any>project).test()

Answer 2

As far as my current knowledge goes, this capability is currently not feasible. There is an ongoing discussion regarding this matter which can be found here: discussion.

Therefore, one option is to refrain from using decorators for extending classes and consider utilizing interfaces with declaration merging for expanding type declarations. Another alternative would be to use type assertion (even though it may compromise type checks): (<any>project).test()

Answer 3

Answer №2

By cleverly combining decorator and mixin techniques, I managed to get everything up and running smoothly.

Start off by defining your own mixin.

PLEASE NOTE: While using a mixin is optional, it provides the advantage of multiple inheritance and increased flexibility.

export function CustomizeMixin<T extends CustomizeConstructor>({ Base }: MixinProps<T>) {

  class Mixin extends Base {

    // Additional decorators can be defined here
    async customMethod() {
      // Your implementation goes here
    }

  }

  return Mixin;
}

// Specify the type of your constructor.
export type Constructor<T, Arguments extends unknown[] = any[]> = new (...arguments_: Arguments) => T;
export type CustomizeConstructor<T = { }> = Constructor<T>;

// Define an interface with the same name as your decorator for proper declaration merging.
export interface CustomizeDecorator { 
  async customMethod(): Promise<void>;
}

// Implement your decorator
export function CustomizeDecorator(options = {}) {

  return function <T extends CustomizeConstructor>(Base: T) {

    // Additional decorators can be defined here
    class CustomizeClass extends CustomizeMixin(Base) { }

    // Transfer original class descriptors to the new class
    const ownPropertyDescriptors = Object.getOwnPropertyDescriptors(Base);

    const { prototype, ...descriptors } = ownPropertyDescriptors;

    Object.defineProperties(CustomizeClass, descriptors);

    return CustomizeClass as T;

  }

}

// Apply the decorator and declare a class interface that extends the decorator interface
// for efficient type merging.
interface CustomService extends CustomizeDecorator {}
@CustomizeDecorator({})
class CustomService {

}

Answer 4

By cleverly combining decorator and mixin techniques, I managed to get everything up and running smoothly.

Start off by defining your own mixin.

PLEASE NOTE: While using a mixin is optional, it provides the advantage of multiple inheritance and increased flexibility.

export function CustomizeMixin<T extends CustomizeConstructor>({ Base }: MixinProps<T>) {

  class Mixin extends Base {

    // Additional decorators can be defined here
    async customMethod() {
      // Your implementation goes here
    }

  }

  return Mixin;
}

// Specify the type of your constructor.
export type Constructor<T, Arguments extends unknown[] = any[]> = new (...arguments_: Arguments) => T;
export type CustomizeConstructor<T = { }> = Constructor<T>;

// Define an interface with the same name as your decorator for proper declaration merging.
export interface CustomizeDecorator { 
  async customMethod(): Promise<void>;
}

// Implement your decorator
export function CustomizeDecorator(options = {}) {

  return function <T extends CustomizeConstructor>(Base: T) {

    // Additional decorators can be defined here
    class CustomizeClass extends CustomizeMixin(Base) { }

    // Transfer original class descriptors to the new class
    const ownPropertyDescriptors = Object.getOwnPropertyDescriptors(Base);

    const { prototype, ...descriptors } = ownPropertyDescriptors;

    Object.defineProperties(CustomizeClass, descriptors);

    return CustomizeClass as T;

  }

}

// Apply the decorator and declare a class interface that extends the decorator interface
// for efficient type merging.
interface CustomService extends CustomizeDecorator {}
@CustomizeDecorator({})
class CustomService {

}

Enhance your TypeScript classes with decorators that can add new methods to your class

Answer №1

Answer №2

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