Resolving NestJs with Fastify: Code Not Executing After app.listen()

Описание: Discover how to resolve the issue of code execution halting after `app.listen()` in your NestJs application when using Fastify.
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This video is based on the question https://stackoverflow.com/q/68643183/ asked by the user 'Marco Moura' ( https://stackoverflow.com/u/16588272/ ) and on the answer https://stackoverflow.com/a/68644014/ provided by the user 'Marco Moura' ( https://stackoverflow.com/u/16588272/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the NestJs and Fastify Conundrum

As developers dive into building applications with NestJs and Fastify, they may encounter unexpected issues that hinder their workflow. One common problem is when code execution simply halts after calling the app.listen() method in the main.ts file. This issue leaves developers in a quandary, especially if they need to run critical code after bootstrapping their application.

In this guide, we'll dissect the problem, explore a likely cause, and guide you through the solution.

The Problem Description

You might find yourself pulling your hair out, wondering why the following lines after app.listen() aren’t executing:

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The surprising part is that the NestJs application operates correctly, but the console.log statement never runs. Many developers have reported this issue, particularly when transitioning from using Express to Fastify.

Delving into the Solution

Thanks to community input, especially from a user named @ Micael Levi, we can draw attention to an underlying cause that usually leads to this execution halt—Double Instance of a controller.

Identifying Double Instances of Controllers

In a typical scenario, you might mistakenly declare the same controller in different module contexts, leading to confusion in the application. Let’s illustrate the error by looking at a common setup.

Example Misconfiguration

Consider the AuthModule where the AuthController is defined:

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In the AppModule, however, the same AuthController is inadvertently imported again:

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

The Resolution

To resolve this problem, simply remove the second declaration of AuthController from the AppModule:

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

Why This Affects Fastify but Not Express

Interestingly, you might wonder why this issue does not occur with Express. Fastify is more stringent regarding duplicate instances and how it manages application contexts, which leads to execution halts when misconfigurations exist. Unfortunately, Express might not raise these issues until runtime, which is why you may not see errors during development.

Conclusion

Avoiding double instance declarations of controllers is a simple yet critical step for ensuring your NestJs application with Fastify runs without a hitch. If you ever find your app.listen() hanging, consider checking your module configurations.

Happy coding, and may your NestJs journey be as smooth as possible!