Implement PipeTransform with string generic types, guard against non-string inputs with a typeof check, and return the sanitized value. Apply at param level for targeted sanitization or at handler level with @UsePipes() for all string arguments of that route.

SanitizeStringPipe implementation and usage

Custom pipe design best practices:

1
Always guard non-target types with typeof or metadata.metatype checks — pass them through unchanged.
2
Use PipeTransform<Input, Output> generics to make the transformation contract explicit.
3
Param-level application is the most precise — only the targeted argument is processed.
4
Handler-level @UsePipes() applies to all arguments — useful for bulk sanitization.
5
Stateless pipes (no injected services) can be used as class references without new.

Difficulty: 5/10

Topics: pipes, validation, string sanitization

Scenario Questions

0-2 years experience

1
Can you walk me through how you'd implement a NestJS pipe that trims whitespace and lowercases a string coming from a request body?
2
If you applied your pipe to a DTO property that sometimes receives null, what would happen and how would you handle it?
3
Where would you register this pipe if you only want it to affect a single controller method?

2-5 years experience

1
Suppose you added this sanitizing pipe globally, but some endpoints expecting case‑sensitive strings (like API keys) are now failing. How would you troubleshoot and resolve the issue?
2
Your team wants the pipe to also reject strings that contain prohibited characters. How would you extend the pipe while keeping it reusable?
3
During testing, the pipe seems to be skipped when validation fails earlier in the pipeline. Explain why that might happen and how to ensure it runs.

5-8 years experience

1
When scaling the service to handle thousands of requests per second, what performance considerations would you evaluate for a globally applied sanitizing pipe?
2
How would you design a strategy to version your custom pipe so that older clients can continue using the previous sanitization behavior while new clients get the updated logic?
3
If multiple modules need slightly different sanitization rules, how would you structure your pipe architecture to avoid code duplication and maintain consistency?

8+ years experience

1
Across several microservices, you need consistent string sanitization but each service uses different NestJS versions. How would you create a shared library for the pipe, handling compatibility and deployment?
2
Your organization is moving from a monolith to a distributed system and wants to enforce input sanitization centrally. Discuss the trade‑offs between implementing the pipe at the API gateway versus inside each NestJS service.
3
What governance processes would you put in place to ensure any new custom pipes meet security, testing, and performance standards across teams?

Follow-up Questions

❓What edge cases did you consider when writing the pipe?
❓How would you test this pipe in isolation and as part of an integration test?
❓Can you discuss any potential security implications of sanitizing inputs this way?