Why middleware processes requests before handlers in NestJS - Visual Breakdown

Concept Flow - Why middleware processes requests before handlers

Incoming HTTP Request

↓

Middleware Processes Request

↓

Pass to Next Middleware or Handler

↓

Route Handler Executes

↓

Response Sent Back

The request first goes through middleware which can modify or check it before passing it to the route handler that sends the response.

Execution Sample

NestJS

app.use((req, res, next) => {
  console.log('Middleware start');
  req.processed = true;
  next();
});
app.get('/', (req, res) => {
  res.sent = true;
  res.send('Hello');
});

Middleware logs a message before the route handler sends 'Hello' as response.

Execution Table

Step	Action	Middleware Log	Handler Called	Response Sent
1	Request received		No	No
2	Middleware runs	Middleware start	No	No
3	Middleware calls next()	Middleware start	Yes	No
4	Handler runs	Middleware start	Yes	No
5	Handler sends response	Middleware start	Yes	Hello

💡 Response sent by handler, request processing ends

Variable Tracker

Variable	Start	After Middleware	After Handler	Final
req.processed	undefined	true (middleware sets flag)	true	true
res.sent	undefined	undefined	true (handler sends)	true

Key Moments - 2 Insights

Why does middleware run before the handler?

Middleware runs first to allow pre-processing like logging or authentication before the handler runs, as shown in execution_table step 2 and 3.

What happens if middleware does not call next()?

If middleware does not call next(), the handler never runs and response is not sent, stopping the flow early (not shown in table but implied).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the handler start running?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Handler Called' column in execution_table rows

According to variable_tracker, when does res.sent become true?

A After Middleware

B After Handler

C At Start

D Never

💡 Hint

Look at the 'res.sent' row in variable_tracker

If middleware skipped calling next(), what would happen to the handler?

A Handler runs anyway

B Handler runs twice

C Handler never runs

D Middleware runs again

💡 Hint

Recall key_moments explanation about next() importance

Concept Snapshot

Middleware runs before route handlers to process requests early.
It can modify requests or block them.
Middleware must call next() to continue flow.
Handlers run after middleware to send responses.
This order ensures control and flexibility.

Full Transcript

In NestJS, when a request comes in, it first passes through middleware. Middleware can do things like logging or checking if the user is allowed. It must call next() to let the request continue. After middleware, the route handler runs and sends the response. If middleware does not call next(), the handler never runs and the request stops there. This order helps keep control over requests and responses.