The document discusses various pitfalls of using middleware in Node.js, emphasizing issues like reference pollution, race conditions, and coupling. It also outlines how to effectively structure middleware to avoid becoming a fat controller and mixing disparate functionalities. Recommendations include maintaining separation of concerns and keeping middleware focused on specific tasks.

1. Node.js Middleware
Never again!
Timur Shemsedinov
Chief Technology Architect at Metarhia
Lecturer at Kiev Polytechnic Institute

2. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
What is middleware?
● Connect contract
(req: Request, res: Response, next: NextFunction)
(req: Request, res: Response)
● Koa contract
async (ctx: Context, next: NextFunction): Promise
async (ctx: Context): Promise

3. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middlewares and Controller
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res)
(req, res, next)
(req, res, next)

4. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
What is middleware?
● Mixin provocation
● Reference pollution and shared state provocation
● Race condition provocation
● Abstraction leak provocation
● Fat controller and layers mix provocation
● High coupling provocation
● Error ignoring provocation

5. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

6. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
req, res

7. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Pass ref outer
const sockets = new Map();
app.use((req, res, next) => {
...
sockets.set(userId, req.socket);
...
next();
});
for (const socket of sockets) {
doSomethingWithSocket(socket);
}

8. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
instance.method(req, res, ...);
instance.method(req.socket, ...);
instance.field = req.headers;
instance.emit('request', req);
const f1 = method.bind(req);
const f2 = method(req)(res);

9. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
const client = new Client(req, res);
abstraction.method(client);
instance.field = client;
client.doSomething(instance);
emitter.emit('connected', client);
func(client).then(...).then(...)...;

10. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
calls

11. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
events
calls

12. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
events
calls
global

13. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)
events
calls
global
fields

14. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

15. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

16. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

17. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

18. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Pass ref everywhere
const ee = new EventEmitter();
app.use((req, res, next) => {
ee.emit('timeout', res);
next();
});
ee.on('timeout', res) => {
setTimeout(() => {
if (!res.writableEnded) res.end('timeout');
}, 5000);
});

19. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Reference pollution
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next)
(req, res)

20. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Race, data corruption
let userId;
app.use((req, res, next) => {
userId = ...;
next();
});
app.get('/resource', (req, res) => {
if (checkAccess('/resource', userId) === GRANTED) {
res.end('You have an access');
}
});

21. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Mixin pollutions
Middleware
Middleware
Middleware(req, res, next)
Controller
(req, res, next)
(req, res, next) (req, res)

22. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: mixin to req, res, ctx
app.use((req, res, next) => {
res.groupName = 'idiots';
next();
});
app.get('/user', (req, res) => {
if (res.groupName === 'idiots') {
res.end('Welcome, my dear friend!');
}
});

23. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: mixin to req, res, ctx
app.use((req, res, next) => {
res.locals.groupName = 'idiots';
next();
});
app.get('/user', (req, res) => {
if (res.locals.groupName === 'idiots') {
res.end('Welcome, my dear friend!');
}
});

24. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Fat controller
Middleware
Middleware
Middleware Controller
● Access db
● Logging
● Parse headers
● Cache
● Business-logic
● Validation
● Serialization
(req, res, next)
(req, res)

25. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Don’t mix in a single function
● Data access (database connection)
● Business-logic and domain model
● Routing, logging, configuration
● Health and server state reporting
● Working with sockets, headers, cookies, etc.
● Serialization and deserialization
● Templating, caching, cryptography, sessions

26. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware: Fat controller
router.get('/user/:id', (req, res) => {
if (blacklist.has(res.socket.remoteAddress)) {...}
const id = parseInt(req.params.id);
if (!isValidateUserId(id)) return res.status(500);
const query = 'SELECT * FROM users WHERE id = $1';
pool.query(query, [id], (err, data) => {
if (err) {...}
logger.write(`access user: ${id}`);
res.status(200).json(data.rows);
});
});

27. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Middleware provocates antipatterns
● Pass-through parameters,
Too many parameters, Data clump
● Accumulate and fire
● Temporary field, State mixins, Shared state,
Global state, State in outer contexts
● Inappropriate intimacy
● High coupling (also see high cohesion pattern)

28. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Layered (onion) architecture
Data Access Layer
Middleware
Middleware
Controller
(req, res, next)
(req, res, next)
(req, res, next)
(req, res)
Service

29. domain
model
app
servicesuse cases
controllers
infrastructure

30. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
We need patterns and principles
● DIP (dependency inversion principle)
IoC (Inversion of control)
DI (dependency injection)
● Chain of responsibility (GoF)
● Law of demeter
● SRP (Single responsibility, SOLID)
● Low coupling (GRASP)

31. const fs = require('fs'); const compose = (...funcs) => x => funcs.
reduce((x, fn) => fn(x), x); const DENSITY_COL = 3; const renderTab
table => { const cellWidth = [18, 10, 8, 8, 18, 6]; return table.ma
=> (row.map((cell, i) => { const width = cellWidth[i]; return i ? c
Keep attention on
● Domain in the middle
● Context isolation
● Layered (onion) architecture
● Don’t depend on frameworks
● Don’t move logic between model and controller
● Always work on abstraction leaking
● Protect data with parallel primitives

32. github.com/tshemsedinov
youtube.com/TimurShemsedinov
github.com/HowProgrammingWorks/Index
Весь курс по ноде и JS (186 лекций)
https://habr.com/ru/post/485294/
t.me/HowProgrammingWorks
t.me/NodeUA
timur.shemsedinov@gmail.com