Middleware ordering in NestJS - Step-by-Step Execution

Concept Flow - Middleware ordering

Request Received

↓

Middleware 1 Executes

↓

Middleware 2 Executes

↓

Route Handler Executes

↓

Response Sent Back

Middleware run in the order they are registered. Each middleware processes the request before passing it on.

Execution Sample

NestJS

app.use(middlewareOne);
app.use(middlewareTwo);

// Request -> middlewareOne -> middlewareTwo -> handler

Shows how two middlewares run in sequence before the route handler.

Execution Table

Step	Middleware	Action	Next Step
1	middlewareOne	Processes request, calls next()	Pass to middlewareTwo
2	middlewareTwo	Processes request, calls next()	Pass to route handler
3	Route Handler	Handles request, sends response	Response sent
4	-	Request cycle complete	End

💡 All middlewares called next(), request reached handler and response sent

Variable Tracker

Variable	Start	After middlewareOne	After middlewareTwo	Final
request.processedBy	[]	[middlewareOne]	[middlewareOne, middlewareTwo]	[middlewareOne, middlewareTwo, handler]

Key Moments - 2 Insights

Why does middlewareTwo run after middlewareOne?

Because middleware are called in the order they are registered, as shown in execution_table steps 1 and 2.

What happens if a middleware does not call next()?

The request stops there and does not reach the next middleware or handler, so response is never sent. This is why calling next() is crucial.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 2?

A middlewareTwo processes request and calls next()

B middlewareOne sends response

C Route handler executes

D Request ends without response

💡 Hint

Check execution_table row with Step 2 for middlewareTwo action

At which step does the route handler execute?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at execution_table row where Middleware is 'Route Handler'

If middlewareOne did not call next(), what would happen?

A middlewareTwo would still run

B Route handler would run immediately

C Request would stop at middlewareOne

D Response would be sent twice

💡 Hint

Refer to key_moments about importance of calling next()

Concept Snapshot

Middleware ordering in NestJS:
- Middleware run in registration order
- Each middleware must call next() to continue
- Request flows through all middlewares before handler
- Missing next() stops request chain
- Order affects processing and response timing

Full Transcript

In NestJS, middleware are functions that run in the order they are registered. When a request comes in, it first passes through the first middleware, which can process or modify the request, then calls next() to pass control to the next middleware. This continues until all middlewares have run, then the route handler executes to send the response. If any middleware does not call next(), the request stops there and the response is not sent. This flow is shown step-by-step in the execution table and variable tracker, illustrating how the request is processed sequentially. Understanding this order helps avoid bugs and ensures middleware behave as expected.