# Exception filters

Danet comes with a built-in exceptions layer which is responsible for processing all unhandled exceptions across an application. When an exception is not handled by your application code, it is caught by this layer, which then automatically sends an appropriate user-friendly response.

Out of the box, this action is performed by a built-in global exception filter, which handles all exceptions. When an exception is unrecognized (does not have statusCode and message properties), the built-in exception filter generates the following default JSON response:

{
  "statusCode": 500,
  "message": "Internal server error!"
}

Danet provides a set of standard exceptions that inherit from the base HttpException and represent many of the most common HTTP exceptions:

• BadRequestException
• UnauthorizedException
• NotFoundException
• ForbiddenException
• NotAcceptableException
• RequestTimeoutException
• ConflictException
• GoneException
• HttpVersionNotSupportedException
• PayloadTooLargeException
• UnsupportedMediaTypeException
• UnprocessableEntityException
• InternalServerErrorException
• NotImplementedException
• ImATeapotException
• MethodNotAllowedException
• BadGatewayException
• ServiceUnavailableException
• GatewayTimeoutException
• PreconditionFailedException

While the base (built-in) exception filter can automatically handle many cases for you, you may want full control over the exceptions layer. For example, you may want to add logging or use a different JSON schema based on some dynamic factors. Exception filters are designed for exactly this purpose. They let you control the exact flow of control and the content of the response sent back to the client.

Let's create an exception filter that is responsible for catching exceptions which are an instance of the CustomException class, and implementing custom response logic for them.

import {
  ExceptionFilter,
  Catch,
  HttpContext,
} from 'https://deno.land/x/danet/mod.ts';

@Injectable()
@Catch(CustomException)
export class CustomExceptionFilter implements ExceptionFilter {
  catch(exception: CustomException, ctx: HttpContext) {
    const response = ctx.response;
    const request = ctx.request;
    const status = exception.status;

    response.status = status;
    response.body = {
      statusCode: status,
      timestamp: new Date().toISOString(),
      path: request.url,
    };
  }
}

The @Catch(CustomException) decorator binds the required metadata to the exception filter, telling Danet that this particular filter is looking for exceptions of type CustomException and nothing else. The @Catch() decorator takes a single parameter.

Let's tie our new CustomExceptionFilter to the TodoController's create() method.

@Post()
@UseFilter(CustomerExceptionFilter)
async create(@Body() createTodoDto: CreateTodoDto) {
  throw new ForbiddenException();
}

We have used the @UseFilters() decorator here. Similar to the @Catch() decorator, it takes the class of your filter, leaving responsibility for instantiation to the framework, and enabling dependency injection.

In the example above, the CustomException is applied only to the single create() route handler, making it method-scoped. Exception filters can be scoped at different levels: method-scoped, controller-scoped, or global-scoped. For example, to set up a filter as controller-scoped, you would do the following:

@UseFilter(CustomExceptionFilter)
export class TodoController {}

This construction sets up the CustomException for every route handler defined inside the TodoController.

In order to catch every unhandled exception (regardless of the exception type), omit @Catch() decorator.

import {
  Catch,
  ExceptionFilter,
  HttpContext,
} from 'https://deno.land/x/danet/mod.ts';

@Injectable()
export class AllExceptionsFilter implements ExceptionFilter {
  constructor() {}

  catch(exception: unknown, ctx: HttpContext): boolean {
    const response = ctx.response;
    const request = ctx.request;
    const status = exception.status;

    response.status = status;
    response.body = {
      statusCode: status,
      timestamp: new Date().toISOString(),
      path: request.url,
    };
  }
}