next() function and flow control in Express - Step-by-Step Execution

Concept Flow - next() function and flow control

Request received

↓

Middleware 1

↓

Calls next()?

↓

Middleware 2

↓

Calls next()?

↓

Middleware 3

↓

Final handler or error handler

The request passes through middleware functions one by one. Each middleware calls next() to pass control to the next middleware. If next() is not called, the flow stops and response is sent.

Execution Sample

Express

app.use((req, res, next) => {
  console.log('First middleware');
  next();
});
app.use((req, res) => {
  res.send('Done');
});

This code logs a message in the first middleware, then calls next() to continue to the next middleware which sends the response.

Execution Table

Step	Middleware	Action	next() Called?	Next Step	Output
1	Middleware 1	Logs 'First middleware'	Yes	Middleware 2	No output yet
2	Middleware 2	Sends response 'Done'	No	Response sent	Response sent: 'Done'
3	End	No more middleware	-	-	Request complete

💡 Middleware 2 does not call next(), so response is sent and flow stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
req	Incoming request object	Passed to Middleware 1	Passed to Middleware 2	Handled
res	Response object	Passed to Middleware 1	Response sent with 'Done'	Closed
next	Function to call next middleware	Called at Step 1	Not called at Step 2	Not called

Key Moments - 2 Insights

Why does the response get sent only after Middleware 2 and not Middleware 1?

Because Middleware 1 calls next(), it passes control to Middleware 2. Middleware 2 sends the response and does not call next(), so the flow stops there (see execution_table Step 2).

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

The request will hang because the flow stops without sending a response or moving forward. This is why calling next() or sending a response is essential in each middleware.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after Step 1?

A No output yet

B 'Done' response sent

C Error message

D Request ended

💡 Hint

Check the 'Output' column for Step 1 in the execution_table.

At which step does the response get sent?

A Step 3

B Step 1

C Step 2

D No response sent

💡 Hint

Look at the 'Action' and 'Output' columns in execution_table rows.

If Middleware 1 did not call next(), what would happen?

A Middleware 2 would still run

B Request would hang without response

C Response would be sent immediately

D An error would be thrown

💡 Hint

Refer to key_moments about what happens if next() is not called and no response is sent.

Concept Snapshot

Express middleware functions run in order.
Each middleware receives (req, res, next).
Calling next() passes control to next middleware.
If next() is not called and no response sent, request hangs.
Sending response ends the flow.
Use next() to chain middleware or handle errors.

Full Transcript

In Express, when a request comes in, it passes through middleware functions one by one. Each middleware can do something with the request or response. To move to the next middleware, it must call the next() function. If next() is not called, the flow stops. Usually, the last middleware sends a response to the client. If a middleware neither calls next() nor sends a response, the request will hang and never finish. This flow control is important to understand to build Express apps that work correctly.