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Learn how to effectively validate object keys in your NestJS TypeScript project using custom validators. Ensure only valid filter parameters pass through your application.
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This video is based on the question https://stackoverflow.com/q/63774957/ asked by the user 'Manspof' ( https://stackoverflow.com/u/6455516/ ) and on the answer https://stackoverflow.com/a/63818277/ provided by the user 'Jay McDoniel' ( https://stackoverflow.com/u/9576186/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Custom Validators in NestJS

When developing applications in NestJS with TypeScript, validating query parameters is crucial for maintaining data integrity and security. For instance, you may want to ensure that only specific keys and values pass validation in your query filters. Here's a scenario to illustrate this concept:

The Problem: Validating Filter Query Parameters

Imagine you're building an endpoint that accepts a filters object in the query parameter. You want to validate that only specific keys (e.g., userIds, ages, names) are allowed and that their corresponding values follow the expected types.

Valid Examples

Good Params:

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Invalid Examples

Bad Params:

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To implement this validation seamlessly in your NestJS application, you'll need to employ some handy features of the class-validator and class-transformer libraries in TypeScript.

The Solution: Using Custom Validators

Let's break down the solution into straightforward steps.

Step 1: Define Your DTOs

First, create a FiltersOptionsDto class to outline the acceptable filter options and their types.

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Step 2: Implement the Main Query DTO

Next, you need to set up the main Data Transfer Object (DTO) to include the filters parameter. This is where the validation and transformation magic happens.

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Step 3: Configure the ValidationPipe

You must configure the ValidationPipe in your AppModule. This helps enforce your validation rules.

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Why This Works

The @ Transform() decorator converts your filter string into JSON before validating.

The @ ValidateNested() ensures that the validation applies to the nested filters object.

Setting forbidNonWhitelisted: true in the ValidationPipe guarantees that any non-defined keys cannot pass through, effectively filtering out unwanted data.

Conclusion

By following these clear steps, you've set up a robust validation mechanism in your NestJS application. Now, your endpoint can effectively handle filter parameters while maintaining strict data validation, ensuring only valid requests are processed successfully.

With this knowledge, you can improve your application’s reliability and security. Embrace the power of custom validators in NestJS, and let your applications shine!