Middleware ordering in NestJS - Performance & Optimization

Performance: Middleware ordering

MEDIUM IMPACT

Middleware order affects how fast requests are processed and how many times the browser waits before getting a response.

Handling HTTP requests with multiple middleware functions

NestJS

app.use(lightMiddleware);
app.use(heavyMiddleware);

Light middleware runs first, quickly handling simple tasks and reducing delay before heavy processing.

📈 Performance GainReduces average request processing time, improving input responsiveness (INP).

Handling HTTP requests with multiple middleware functions

NestJS

app.use(heavyMiddleware);
app.use(lightMiddleware);

Heavy middleware runs before lightweight ones, causing unnecessary delay for all requests.

📉 Performance CostBlocks request processing longer, increasing input delay (INP) for users.

Performance Comparison

Pattern	Middleware Order	Request Delay	User Impact	Verdict
Heavy middleware first	Heavy → Light	High delay	Slower response, worse INP	[X] Bad
Light middleware first	Light → Heavy	Lower delay	Faster response, better INP	[OK] Good

Rendering Pipeline

Middleware runs on the server before sending a response. The order affects how quickly the server can start sending data back to the browser.

→Request Handling

→Response Preparation

⚠️ BottleneckRequest Handling stage due to synchronous or heavy middleware blocking the pipeline.

Core Web Vital Affected

INP

Middleware order affects how fast requests are processed and how many times the browser waits before getting a response.

Optimization Tips

1Place lightweight middleware before heavy middleware to reduce request delay.

2Avoid blocking operations early in middleware to improve input responsiveness (INP).

3Use asynchronous middleware wisely to prevent blocking the request pipeline.

Performance Quiz - 3 Questions

Why should lightweight middleware run before heavy middleware in NestJS?

A To delay response so the server can prepare more data

B To reduce request processing time and improve input responsiveness

C To increase bundle size for better caching

D To avoid using asynchronous code

DevTools: Network and Performance panels

How to check: Record a performance profile while making requests; check waterfall timings and server response times.

What to look for: Look for long server processing times before response starts, indicating middleware blocking.

Practice

1. In NestJS, why is the order of middleware important when using the apply() method?

easy

A. Because middleware order controls the type of HTTP methods allowed.

B. Because middleware order changes the port number the server listens on.

C. Because middleware order determines the database connection settings.

D. Because middleware runs in the order they are applied, affecting request processing sequence.

Solution

1. Step 1: Understand middleware execution flow

   Middleware functions run in the order they are applied to the request pipeline.

2. Step 2: Recognize impact on request handling

   The order affects how requests are logged, authenticated, or modified before reaching the route handler.

3. Final Answer:

   Because middleware runs in the order they are applied, affecting request processing sequence. -> Option D

4. Quick Check:

   Middleware order = request handling sequence [OK]

Hint: Middleware order controls request flow sequence [OK]

Common Mistakes:

• Thinking middleware order changes server port
• Confusing middleware order with database settings
• Assuming middleware order controls HTTP methods

2. Which of the following is the correct way to apply two middleware functions authMiddleware and loggerMiddleware in order using NestJS apply() method?

easy

A. apply([authMiddleware, loggerMiddleware]).forRoutes('*')

B. apply(loggerMiddleware, authMiddleware).forRoutes('*')

C. apply(authMiddleware, loggerMiddleware).forRoutes('*')

D. apply(loggerMiddleware).apply(authMiddleware).forRoutes('*')

Solution

1. Step 1: Recall correct syntax for multiple middleware

   Use apply(middleware1, middleware2) to apply middleware in order.

2. Step 2: Confirm order matches desired sequence

   To run authMiddleware before loggerMiddleware, list authMiddleware first.

3. Final Answer:

   apply(authMiddleware, loggerMiddleware).forRoutes('*') -> Option C

4. Quick Check:

   apply(mw1, mw2) = correct syntax [OK]

Hint: List middleware in apply() in desired run order [OK]

Common Mistakes:

• Reversing middleware order in apply()
• Using multiple apply() calls incorrectly
• Passing middleware as an array inside apply()

3. Given the following middleware setup in NestJS:

apply(authMiddleware, loggerMiddleware).forRoutes('users')

What will be the order of middleware execution when a request hits the 'users' route?

medium

A. authMiddleware runs first, then loggerMiddleware

B. loggerMiddleware runs first, then authMiddleware

C. Both run simultaneously

D. Only authMiddleware runs

Solution

1. Step 1: Identify middleware order in apply()

   Middleware listed first runs first; here, authMiddleware is first.

2. Step 2: Confirm sequence on route match

   When a request matches 'users', authMiddleware runs before loggerMiddleware.

3. Final Answer:

   authMiddleware runs first, then loggerMiddleware -> Option A

4. Quick Check:

   apply order = execution order [OK]

Hint: First middleware in apply() runs first [OK]

Common Mistakes:

• Assuming middleware run simultaneously
• Thinking last middleware runs first
• Ignoring route matching

4. You wrote this middleware setup in NestJS:

apply(loggerMiddleware, authMiddleware).forRoutes('profile')

But you want authMiddleware to run before loggerMiddleware. What is the fix?

medium

A. Use apply(authMiddleware).apply(loggerMiddleware).forRoutes('profile')

B. Swap the middleware order to apply(authMiddleware, loggerMiddleware).forRoutes('profile')

C. Change forRoutes to forRoutes('auth')

D. No fix needed; order does not matter

Solution

1. Step 1: Identify current middleware order

   Currently, loggerMiddleware runs before authMiddleware.

2. Step 2: Swap middleware order in apply()

   To run authMiddleware first, list it first in apply().

3. Final Answer:

   Swap the middleware order to apply(authMiddleware, loggerMiddleware).forRoutes('profile') -> Option B

4. Quick Check:

   Middleware order in apply() = execution order [OK]

Hint: Swap middleware order in apply() to fix sequence [OK]

Common Mistakes:

• Changing route instead of middleware order
• Using multiple apply() calls incorrectly
• Assuming order does not affect execution

5. You want to log every request before authenticating it and then modify the request body only after authentication in NestJS. Which middleware order should you apply?

hard

A. apply(loggerMiddleware, authMiddleware, modifyBodyMiddleware).forRoutes('*')

B. apply(authMiddleware, loggerMiddleware, modifyBodyMiddleware).forRoutes('*')

C. apply(modifyBodyMiddleware, loggerMiddleware, authMiddleware).forRoutes('*')

D. apply(authMiddleware, modifyBodyMiddleware, loggerMiddleware).forRoutes('*')

Solution

1. Step 1: Determine desired middleware sequence

   Requests should be logged first, then authenticated, then modified.

2. Step 2: Arrange middleware in apply() accordingly

   List loggerMiddleware first, then authMiddleware, then modifyBodyMiddleware.

3. Final Answer:

   apply(loggerMiddleware, authMiddleware, modifyBodyMiddleware).forRoutes('*') -> Option A

4. Quick Check:

   Middleware order = log -> auth -> modify [OK]

Hint: Order middleware as log, auth, then modify [OK]

Common Mistakes:

• Placing modifyBodyMiddleware before authMiddleware
• Logging after authentication
• Incorrect middleware sequence in apply()