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Learn how to effectively validate an array of objects using TypeScript, class validators, and middleware in Node.js. This post covers a recursive solution to handle nested validation errors.
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How to Validate an Array of Objects in Node.js with TypeScript

When developing web applications with Node.js and TypeScript, one common requirement is to validate incoming data—especially when handling arrays of objects that represent structured data. In this guide, we'll explore how to validate an array of objects through a real-world example, particularly addressing an issue where deeply nested validation errors can complicate debugging.

The Problem

Imagine you're working with a validation schema for a fee structure, which includes multiple objects that each have nested properties. Here’s a representation of the data you might receive through an API request via Postman:

[[See Video to Reveal this Text or Code Snippet]]

For this data structure, you may already have a validation schema defined using class validators in TypeScript:

[[See Video to Reveal this Text or Code Snippet]]

In your middleware function, designed to validate this request body, you might face an issue where validation errors are structured in a nested manner. Specifically, the @ ValidateNested() decorator does not directly return the error details; instead, errors are nested, leading to an unexpected structure:

[[See Video to Reveal this Text or Code Snippet]]

This can become quite cumbersome if there are multiple levels of nesting, complicating how you handle error messages.

The Solution

To resolve this issue effectively, we can create a recursive function that traverses the error object and gathers all the relevant constraints, regardless of how many levels of nesting there are. Here’s how you can achieve this:

Step 1: Updating Your Middleware

You should modify your validation middleware to include a helper function that can process errors recursively:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Implementing Recursive Error Handling

Here's the key function, getErrorMessages, that will traverse through the errors:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Recursive Function

Initialization: We create an empty array messages to store any messages we accumulate.

Iterating through Errors: For each error, we first check if there are any direct constraints. If there are, we add them to messages.

Handling Nested Errors: If there are child errors (indicating nested validation issues), we recursively call getErrorMessages, gathering those messages as well.

Returning Results: The function compiles messages from all levels of nesting and returns a flat list.

Conclusion

By implementing this recursive approach to error handling in your validation middleware, you can effectively gather all error messages from nested structures without worrying about the depth of nesting. This not only streamlines the validation process but also simplifies debugging and improves the clarity of error messages returned to clients.

So the next time you're faced with complex nested validation scenarios in your Node.js application, remember this approach to keep your code clean and maintainable.