Missing @Injectable() or @Inject() decorators silently injects undefined

[glebbash]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

Missing @Injectable() or @Inject() decorators silently injects undefined.

This is a major issue as it does not prevent Nest.js application from starting and it's very easy to miss this in tests when mocking methods that use these broken injectables.

Notice the log output in the following repro:

Broken	Working
Case 1	Case 2 Case 3

Case 1 is unexpected and should not happen / everyone needs to be aware of this problem.

Minimum reproduction code

https://github.com/glebbash/nest-injectable-repro

Steps to reproduce

1. npm ci
2. npm start
3. Check logs

Expected behavior

There should be an error thrown, stopping the application from initializing or this issue must be documented.

Package

• I don't know. Or some 3rd-party package
• @nestjs/common
• @nestjs/core
• @nestjs/microservices
• @nestjs/platform-express
• @nestjs/platform-fastify
• @nestjs/platform-socket.io
• @nestjs/platform-ws
• @nestjs/testing
• @nestjs/websockets
• Other (see below)

Other package

No response

NestJS version

^10.3.1

Packages versions

[System Information]
OS Version     : Linux 6.1.91-060191-generic
NodeJS Version : v20.15.1
NPM Version    : 10.7.0 

[Nest CLI]
Nest CLI Version : 10.3.0 

[Nest Platform Information]
platform-express version : 10.4.1
schematics version       : 10.1.0
terminus version         : 10.2.2
swagger version          : 7.2.0
testing version          : 10.3.1
common version           : 10.3.1
axios version            : 3.0.1
core version             : 10.3.1
cli version              : 10.3.0

[Warnings]
The following packages are not in the same minor version
This could lead to runtime errors
* Under version 10.4
- @nestjs/platform-express 10.4.1
* Under version 10.3
- @nestjs/common 10.3.1
- @nestjs/core 10.3.1
* Under version 10.1
- @nestjs/schematics 10.1.0

Node.js version

20

In which operating systems have you tested?

• macOS
• Windows
• Linux

Other

No response

[micalevisk]

this is expected due to TypeScript introspection limitations. I believe that you can use ESLint with plugins like https://github.com/darraghoriordan/eslint-plugin-nestjs-typed/ to detect such cases.

Neither @Inject() nor @Injectable() are mandatory if you won't inject anything on the provider, which is why nestjs itself couldn't prevent it.

[glebbash]

But this lint plugin is not mentioned in the docs and does not come with default Nest.js installation.

@micalevisk I don't think it's very user friendly to not document known issues and just give link once to some third party solution in case someone stumbles upon this problem.

[micalevisk]

Got you. Few free to open a PR on the docs github repo to document that. There is a PR open already that covers such topic but maybe having a 'known limitations' page would be better

[jmcdo29]

For what it's worth, there is a PR that I want to have merged that talked about what the @Injectable() decorator does and why it's important

[glebbash]

Linking this one because this issue happened before: #13059.

I might find some time to submit docs PR later.