Injectable decorator in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @Injectable() decorator in NestJS?

The <code>@Injectable()</code> decorator marks a class as a provider that can be managed by NestJS's dependency injection system. It allows the class to be injected into other classes as a dependency.

beginner

Can a class without <code>@Injectable()</code> be injected as a provider in NestJS?

No. Without the <code>@Injectable()</code> decorator, NestJS does not recognize the class as a provider and will not inject it into other classes.

beginner

How does @Injectable() help with managing dependencies in NestJS?

It tells NestJS to create and manage an instance of the class automatically. This way, you don't have to manually create instances or manage dependencies yourself.

intermediate

Is it possible to add metadata or options inside the @Injectable() decorator?

Yes. You can pass options to @Injectable() to control the provider's scope, like making it singleton or request-scoped.

beginner

What happens if you forget to add @Injectable() on a service you want to inject?

NestJS will throw a runtime error saying it cannot resolve the dependency because the class is not registered as a provider.

What does the @Injectable() decorator do in NestJS?

A Registers a middleware

B Defines a controller route

C Creates a database connection

D Marks a class as a provider for dependency injection

@Injectable() marks a class as a provider so NestJS can inject it where needed.

Can you inject a class without @Injectable() in NestJS?

A Yes, always

B Only if it is a controller

C No, it must have <code>@Injectable()</code>

D Only if it extends another class

Classes must have @Injectable() to be injected as providers.

Which of these is a valid use of @Injectable() options?

A Setting the provider scope to singleton

B Defining HTTP routes

C Connecting to a database

D Styling components

You can set provider scope like singleton or request-scoped using options in @Injectable().

What error occurs if you inject a class without @Injectable()?

A Cannot resolve dependency error

B Syntax error

C Type error

D No error

NestJS throws a 'Cannot resolve dependency' error at runtime.

Where should you place the @Injectable() decorator?

A On HTML templates

B On classes you want to inject as providers

C On environment configuration files

D On database schemas

@Injectable() goes on classes to make them injectable providers.

Explain in your own words what the @Injectable() decorator does in NestJS and why it is important.

Think about how NestJS knows which classes to create and share automatically.

Describe what happens if you try to inject a class without the @Injectable() decorator in NestJS.

Consider what NestJS needs to know to inject a class.