Middleware factory pattern in Express - Step-by-Step Execution

Concept Flow - Middleware factory pattern

Call factory function with config

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Factory returns middleware function

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Express uses middleware in request flow

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Middleware runs with access to config

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Next middleware or response

The factory function creates a middleware configured by input, which Express then runs during requests.

Execution Sample

Express

function logger(level) {
  return function(req, res, next) {
    console.log(`[${level}] ${req.method} ${req.url}`);
    next();
  };
}
app.use(logger('INFO'));

Creates a logger middleware with a level, logs request info, then calls next middleware.

Execution Table

Step	Action	Input/State	Output/Result
1	Call logger factory with 'INFO'	level = 'INFO'	Returns middleware function with level='INFO'
2	Express receives request GET /home	req.method='GET', req.url='/home'	Middleware function called with req, res, next
3	Middleware logs message	level='INFO', req.method='GET', req.url='/home'	Console logs: [INFO] GET /home
4	Middleware calls next()	next function	Express continues to next middleware or route handler
5	Request handled or response sent	No more middleware	Response sent to client

💡 Middleware calls next(), allowing Express to continue request processing or send response.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
level	undefined	'INFO'	'INFO'	'INFO'	'INFO'
middleware function	undefined	function(req,res,next)	function(req,res,next)	function(req,res,next)	function(req,res,next)
req.method	undefined	undefined	'GET'	'GET'	'GET'
req.url	undefined	undefined	'/home'	'/home'	'/home'

Key Moments - 2 Insights

Why does the factory function return another function instead of doing the work immediately?

The factory returns a middleware function so Express can call it later with request info. See step 1 and 2 in execution_table where the factory returns a function, and Express calls it with req, res, next.

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

Express will stop processing further middleware or routes, causing the request to hang. Step 4 shows calling next() lets Express continue.

Visual Quiz - 3 Questions

Look at the execution_table, what is logged to the console at step 3?

A [ERROR] GET /home

B [INFO] GET /home

C [INFO] POST /home

D [DEBUG] GET /home

💡 Hint

Check the 'Output/Result' column in step 3 of execution_table.

At which step does Express call the middleware function returned by the factory?

A Step 4

B Step 1

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column describing when middleware is called.

If the factory was called with 'ERROR' instead of 'INFO', what would change in the execution_table?

A The logged message at step 3 would show [ERROR] instead of [INFO]

B The middleware would not be called

C The request method would change

D Nothing would change

💡 Hint

Refer to the 'Input/State' and 'Output/Result' columns at step 3.

Concept Snapshot

Middleware factory pattern:
- A function that takes config and returns middleware
- Middleware runs later with req, res, next
- Allows customizing middleware behavior
- Must call next() to continue Express flow
- Example: logger(level) returns middleware logging requests

Full Transcript

The middleware factory pattern in Express means writing a function that takes some settings and returns a middleware function. This returned middleware runs when Express handles a request. For example, a logger factory takes a level like 'INFO' and returns a middleware that logs each request with that level. When a request comes in, Express calls the middleware with request and response objects. The middleware logs the message and calls next() to let Express continue. This pattern helps customize middleware behavior easily. Key points: the factory returns a function, middleware runs later, and next() must be called to avoid stopping the request.