DTO pattern, class-validator , class-transformer , built-in Pipes,

custom Pipes, and global ValidationPipe setup

These notes explain how NestJS receives data from a request, checks

whether that data is valid, and converts it into the format your

application expects.

The examples use a simple API for creating and fetching posts. You can

adapt the same ideas to users, profiles, authentication, or other

modules.

1. Why do we need validation?

Imagine your API has an endpoint that creates a post. It expects a

request body like this:

{ "content" : "Learning NestJS" , "visibility" : "public" }

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But a client might accidentally send:

{ "content" : 12345 , "visibility" : "everyone" }

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Without validation, incorrect data may travel deeper into your

application and cause unexpected behaviour or database errors.

Validation helps us check incoming data at the API boundary.…