The goal: I want to associate a method with an action value passed through the method decorator and keep this action-method map on the instance of the class whose method was decorated. In other words, I want to “register” a method into a “table” where each method is associated with an action value, for example, { "send": sendMessage(), ... } and do it through the method decorators.

The problem:
When I add a class property for keeping action-method pairs, it’s always empty even when I update it in the decorator. Removing the property “solves” the problem, and the code compiles and runs successfully, but then I lose type checking.

How do I fix this? Is there a better approach?

Here is the working code example and a link to TypeScript Playground:

enum Action {
  Send,
  Create,
};

function action(action: Action) {
  return function <This, Args extends any[], Return>(
    target: (this: This, ...args: Args) => Return,
    context: ClassMethodDecoratorContext<This, (this: This, ...args: Args) => Return>
  ) {
    if (context.kind !== 'method')
      throw new Error('This decorator can only be applied to methods.');

    context.addInitializer(function () {
      if (!this.actionsMap)
        this.actionsMap = new Map();

      this.actionsMap.set(action, target.bind(this));
    });
  };
}

class Service {
  // Results in an empty map:
  // actionsMap: Map<Action, Function> = new Map();

  @action(Action.Send)
  public sendMessage() {
    console.log('Sending message...');
  }

  @action(Action.Create)
  public createMessage() {
    console.log('Creating message...');
  }
}

const service = new Service();
service.actionsMap.get(Action.Send)();