A Cheat Sheet for the Nest JS Request Pipeline

I'm always checking the NestJS documentation to read what a Middleware Vs Interceptor is and I could never remember the order that the different NestJS pipeline elements are called in.

So I made this cheatsheet for myself so I had something to refer to.

The NestJS request pipeline cheatsheet

Click to open full size image - more explanation of the cheat is available below.

In this cheatsheet show the pipeline of Middleware -> Guards -> Before Interceptors -> Pipes -> Controllers -> Services -> After Interceptors -> Exception Filters that you would typically use in a NestJS rest application.

It's important to note that some of these are just highly focused instances of middleware or interceptors. The middleware is the most generic pipeline context and the others are just NestJS constructs to encourage convention.

This isn't 100% true because as the request goes through the pipeline more context is added and made available to you in the specific element. But I think this is a good way to think about these extra NestJS pipeline classes.

I also show where the NestJS exception zone edges are and how Decorators are related to controllers.

I add some side information about NestJS module and application hooks.

Finally there is a list of the handy TypeORM hooks if you use TypeORM.

The Nest JS Request Pipeline

These elements are called in this order. It might be easier to see the relations in the cheatsheet diagram.

Middleware

USED FOR

Modifying or enhancing the request and response objects

Ending the request/response cycle

NOTES

Can be used to access the request and return a response.

You can’t access the result of a route.

If you return early you will bypass the rest of the NestJS pipeline

CODE

implements NestMiddleware interface

Guards

USED FOR

Used to allow or prevent access to a route

This is where you should authorise requests

NOTES

You can access the execution context here. You can check which Controller will be used

CODE

implements CanActivate interface

Before Interceptor

USED FOR

Before and after enhancing of any part of the request-response

e.g. logging or caching

NOTES

Interceptors can access the request and response

CODE

implements NestInterceptor

Pipes

USED FOR

Transforming input data

Validating input data

NOTES

Pipes should operate on the arguments to be passed to controller route.

There are a bunch of built in Pipes. ValidationPipe is very useful.

CODE

implements PipeTransform interface

Decorators

USED FOR

Extracting request data into a Controller method parameter

e.g. if you set the user using passport you could extract it to a custom decorator like @MyAppUser user: MyAppUser instead of doing request?.user? every time.

NOTES

Has onModuleInit() and onModuleDestroy() hooks

Can be instantiated asynchronously

CODE

decorated with @Module()

Controllers

USED FOR

Route handling methods

NOTES

Decorators can be applied to controllers to extract parameters from the request

CODE

decorated with @Controller()

Your Services

USED FOR

Custom business logic

NOTES

These are injected via providers.

CODE

decorated with @Injectable()

After Interceptor

USED FOR

Before and after enhancing of any part of the request-response

e.g. logging or caching

NOTES

Interceptors can access the request and response

This block represents where the interceptor handler activates BEFORE the route handler is called

CODE

implements NestInterceptor interface

Exception Filters

USED FOR

Handle thrown Errors and convert into relevant http response

NOTES

There is a built in exception filter that handles the various Nest HttpExceptions that can be thrown in your code

Exception Filters only handle exceptions thrown in the exception zone

CODE

implements ExceptionFilter interface

Additional useful NestJS hooks

Application Hooks

USED FOR

Running code in a service on application start or exit

NOTES

Turn on before app.listen() with app.enableShutdownHooks()

Use in injected service with onApplicationShutdown(signal:string)

CODE

see notes

Modules

USED FOR

Bounding and decoupling a set of domain functionality

NOTES

Has onModuleInit() and onModuleDestroy() hooks

Can be instantiated asynchronously

CODE

decorated with @Module()

Type ORM Hooks

USED FOR

Running code in a service on application start or exit

NOTES

@AfterLoad
@BeforeInsert, @AfterInsert
@BeforeUpdate, @AfterUpdate
@BeforeRemove, @AfterRemove

CODE

only works on classes decorated with @Entity

Conclusion

Hope it helps to see these all in one place!

If you have any feedback let me know on twitter!