# Execution context

ExecutionContext provides additional details about the current execution process. Danet provides an instance of ExecutionContext in places you may need it, such as in the canActivate() method of a guard and the action() method of a middleware. It provides the following methods:

type ExecutionContext = {
  /**
   * Returns the type of the controller class which the current handler belongs to.
   */
  getClass(): Constructor;
  /**
   * Returns a reference to the handler (method) that will be invoked next in the
   * request pipeline.
   */
  getHandler(): Function;
}

The getHandler() method returns a reference to the handler about to be invoked. The getClass() method returns the type of the Controller class which this particular handler belongs to. For example, if the currently processed request is a POST request, bound to the create() method on the TodoController, getHandler() returns a reference to the create() method and getClass() returns the TodoController type (not instance).

const methodKey = ctx.getHandler().name; // "create"
const className = ctx.getClass().name; // "TodoController"

The ability to access references to both the current class and handler method provides great flexibility. Most importantly, it gives us the opportunity to access the metadata set through the @SetMetadata() decorator from within guards or interceptors. We cover this use case below.

Danet provides the ability to attach custom metadata to route handlers through the @SetMetadata() decorator. We can then access this metadata from within our class to make certain decisions.

@Post()
@SetMetadata('roles', ['admin'])
async create(@Body() createTodoDto: CreateTodoDto) {
  this.todoService.create(createCatDto);
}

With the construction above, we attached the roles metadata (roles is a metadata key and ['admin'] is the associated value) to the create() method. While this works, it's not good practice to use @SetMetadata() directly in your routes. Instead, create your own decorators, as shown below:

import { SetMetadata } from 'https://deno.land/x/danet/mod.ts';

export const Roles = (...roles: string[]) => SetMetadata('roles', roles);

This approach is much cleaner and more readable, and is strongly typed. Now that we have a custom @Roles() decorator, we can use it to decorate the create() method.

@Post()
@Roles('admin')
async create(@Body() createTodoDto: CreateTodoDto) {
    this.todoService.create(createCatDto);
}

To access the route's role(s) (custom metadata), we'll use the MetadataHelper static methods,

To read the handler metadata, use the get() method.

const roles = MetadataHelper.getMetadata<string[]>('roles', context.getHandler());

The MetadataHelper#getMetadata method allows us to easily access the metadata by passing in two arguments: a metadata key and a context (decorator target) to retrieve the metadata from. In this example, the specified key is 'roles' (refer back to the roles.decorator.ts file above and the SetMetadata() call made there). The context is provided by the call to context.getHandler(), which results in extracting the metadata for the currently processed route handler. Remember, getHandler() gives us a reference to the route handler function.

Alternatively, we may organize our controller by applying metadata at the controller level, applying to all routes in the controller class.

@Roles('admin')
@Controller('todo')
export class TodoController {}

In this case, to extract controller metadata, we pass context.getClass() as the second argument (to provide the controller class as the context for metadata extraction) instead of context.getHandler():

const roles = MetadataHelper.getMetadata<string[]>('roles', context.getClass());