Decorators were proposed in ECMAScript 2016 (https://github.com/wycats/javascript-decorators). They are similar to Java annotations--they also add metadata to class declaration, method, property, and the function's parameter, but they are more powerful. They add new behaviors to their targets. With decorators, we can run arbitrary code before, after, or around the target execution, like in aspect-oriented programming, or even replace the target with a new definition. In TypeScript, you can decorate constructors, methods, properties, and parameters. Every decorator begins with the @ character followed by the name of the decorator.

How does it work under the hood that takes its target as argument? Let's implement a classic example with a logging functionality. We would like to implement a method decorator @log. A method decorator accepts three arguments: an instance of the class on which the method is defined, a key for the property, and the property descriptor (https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty).

If the method decorator returns a value, it will be used as a new property descriptor for this method:

const log = (target: Object, key: string | symbol, descriptor: PropertyDescriptor) => {
  // save a reference to the original method
  var originalMethod = descriptor.value;
  // replace the original function
  descriptor.value = function(...args: any[]) {
    console.log("Arguments: ", args.join(", "));
    const result = originalMethod.apply(target, args);
    console.log("Result: ", result);
    return result;
  }
  return descriptor;
}

class Rectangle {
  @log
  area(height: number, width: number) {
    return height * width;
  }
}

let rect = new Rectangle();
let area = rect.area(2, 3);

This decorator logs received arguments and return values. Decorators can be composed and customized with parameters too. You can write the following, for instance:

class Rectangle {
  @log("debug")
  @persist("localStorage")
  area(height: number, width: number) {
    return height * width;
  }
}

Angular offers different types of decorators that are used for dependency injection or adding metadata information at compilation time:

• Class decorators such as @NgModule, @Component, and @Injectable
• Property decorators such as @Input and @Output
• Method decorators such as @HostListener
• Parameter decorators such as @Inject

TypeScript compiler is able to emit some design-time type metadata for decorators. To access this information, we have to install a Polyfill called reflect-metadata:

npm install reflect-metadata --save

Now we can access, for example, the type of the property (key) on the target object as follows:

let typeOfKey = Reflect.getMetadata("design:type", target, key);