Inspired by the AspectJ java framework, AspectJS leverages ES Decorators to bring Aspect Oriented Programming to Javascript and Typescript.

ECMAScript Decorators are fantastic: they allow developers to hide boilerplate code behind a simple @ sign, keeping the code clean, easy to read, and easy to write. Widely used in popular projects such as Angular, Nest.js or TypeORM, decorators have one major drawback: they come with their own built-in behavior, making interoperability between tools difficult, and preventing them from being repurposed for other uses.

AspectJS takes a different approach, by introducing two important concepts: Annotations and Aspects.

• An Annotation is essentially an empty decorator that marks a target (class, property, method or parameter) as a candidate for further enhancements.
• Aspects can be selectively enabled to introduce new behaviors into the annotated elements.

• without aspects

  const A = function( target: Function) {
     // behaviorA
  }
  const B = function( target: Function) {
     // behaviorB
  }
   
  @A() 
  @B()
  class MyClass {}

• with aspects

  const af = new AnnotationFactory('my.org')
  const A = af.create('A');
  const B = af.create('B');
  
  @A()
  @B()
  class MyClass {}
  
  @Aspect()
  class AB_Aspect {
    // behavior A
    // behavior B
  }
  
  getWeaver().enable(new AB_Aspect())

• Install the package

  npm i @aspectjs/core @aspectjs/common

• Create an annotation:

  // toasted.annotation.ts
  import { AnnotationFactory, AnnotationKind } from '@aspectjs/common';
  
  const ANNOTATION_FACTORY = new AnnotationFactory('demo');
  const Toasted = ANNOTATION_FACTORY.create(
    AnnotationKind.METHOD,
    function Toasted() {},
  );

• Use that annotation on a class, a property, a method or a parameter:

  // main.ts
  class Main {
    @Toasted()
    run() {
      console.log('run');
    }
  }

• Declare an aspect triggered by the annotation:

  // toasted.aspect.ts
  import { Around, AroundContext, Aspect, JoinPoint, on } from '@aspectjs/core';
  @Aspect()
  class ToastedAspect {
    @Around(on.methods.withAnnotations(Toasted))
    toast(ctxt: AroundContext, jp: JoinPoint, jpArgs: unknown[]) {
      const result = jp(...jpArgs);
      const text = `${ctxt.target.label} completed successfully`;
      showToast(text);
      return result;
    }
  }

• Enable the aspect

  // aop.ts
  import { getWeaver } from '@aspectjs/core';
  
  getWeaver().enable(new ToastedAspect());

For more advanced usage, please read the documentation: https://aspectjs.gitlab.io/.

MIT Licensed