Middleware execution flow (req, res, next) in Express - Step-by-Step Execution

Concept Flow - Middleware execution flow (req, res, next)

Request Received

↓

Middleware 1

↓

Calls next()

→Middleware 2

↓

Calls next()

→Middleware 3

↓

Sends Response

↓

Response Sent

The request passes through each middleware in order. Each middleware can call next() to pass control to the next one or send a response to end the chain.

Execution Sample

Express

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

Two middlewares run in order; the first logs and calls next(), the second logs and sends a response.

Execution Table

Step	Middleware	Action	next() Called?	Response Sent?	Console Output
1	Middleware 1	Logs 'First middleware'	Yes	No	First middleware
2	Middleware 2	Logs 'Second middleware' and sends response	No	Yes	Second middleware
3	End	Response sent, no further middleware	N/A	Yes	N/A

💡 Response sent in Middleware 2, so middleware chain stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
req	Incoming request object	Passed to Middleware 2	Passed to Middleware 2	Used to send response
res	Response object	Not sent	Response sent	Response sent
next	Function to call next middleware	Called	Not called	N/A

Key Moments - 3 Insights

Why does the second middleware run after the first?

Because the first middleware calls next(), which passes control to the next middleware as shown in execution_table step 1.

What happens if a middleware sends a response without calling next()?

The middleware chain stops immediately, no further middleware runs, as seen in execution_table step 2 and exit_note.

Can next() be called multiple times in one middleware?

No, calling next() more than once can cause errors or unexpected behavior. Each middleware should call next() once or send a response.

Visual Quiz - 3 Questions

Look at the execution_table, what is the console output after step 1?

A 'First middleware'

B 'Second middleware'

C 'Done'

D No output yet

💡 Hint

Check the 'Console Output' column for step 1 in execution_table.

At which step does the response get sent?

A Step 1

B Step 3

C Step 2

D No response sent

💡 Hint

Look at the 'Response Sent?' column in execution_table.

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

A Response would be sent immediately

B Middleware chain would stop after Middleware 1

C Middleware 2 would still run

D An error would occur

💡 Hint

Refer to the concept_flow and key_moments about next() usage.

Concept Snapshot

Middleware in Express runs in order.
Each middleware gets req, res, next.
Call next() to pass control to next middleware.
Send response to end chain.
If next() not called and no response sent, request hangs.

Full Transcript

In Express, middleware functions run one after another when a request comes in. Each middleware receives three things: the request object, the response object, and a next function. When a middleware finishes its job, it can call next() to let the next middleware run. If it sends a response instead, the chain stops there. This flow ensures that requests are handled step-by-step, and responses are sent only once. The execution table shows how the first middleware logs a message and calls next(), then the second middleware logs another message and sends the response, stopping the chain.