If you have worked in Java universe with Spring frameworks (or others like lombok), you would be familiar with power of annotations.

Put a @RestMapping annotation on a method! Bam! It's exposed as an API.
Put a @Data annotation on a class! Bam! You have all the verbose methods like getters, setters, toString and much more.

Decorators works in a similar way for TypeScript. These 2 enable us to abstract out redundant and non business related codes like caching, logging, validity of parameters and a lot more.

What are Decorators in Typescript

From TypeScriptLang

A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators use the form @expression, where expression must evaluate to a function that will be called at runtime with information about the decorated declaration.

Why to use Decorators

1. Abstract out redundant code
2. Keep business logic clean
3. Boost developer productivity
4. Keep Non-functional things like caching/logging/validations standardise across services

Look at this basic example

const getUsernameById(String userId) {
  const userName = CACHE.get("bucket", userId);
  if(!_.isNil(userName)) {
    return userName;
  }

  const user = UserModel.getById(userId);
  CACHE.set("bucket", userId, user.name);
  Log.info(
    method: "getUsernameById"
    key_1: "userId",
    key_1_value: userId,
    key_2: "userName",
    key_2_value: userName
  )
  return user.name;
}

Using decorators, it can be reduced to

@Cacheable
@Loggable
const getUsernameById(String userId){
  const user = UserModel.getById(userId);
  return user.name;
}

PS: These implementations/naming can be different for others, but gist is same

What all entity Decorators can support

Just like Java, typescript support class method/function and property level decorators and many more:

1. Class decorators: Can be used for creating components or singleton objects.
2. Method decorators: Can be used for logging request response of a method, caching params and returned value, etc
3. Property decorators: Can be used for validating a property value on assignment or initialisation A detailed description can be found here.

How can I start with Decorators for my project

Trust me when I say this, it's very easy to implement. It'll be one of those projects which will give you the highest ROI on engineering investments. It took me few hours to implement @Cache and @Log for a test project. The working project can be found here.

What's next

To create an npm library which out of the box will support these annotations just like spring-boot. Do give a pull request if you have any suggestions.