Custom providers in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom provider in NestJS?

A custom provider in NestJS is a way to define how a dependency is created or supplied, allowing you to control the instance or value injected into your classes.

beginner

How do you define a custom provider using the 'useValue' syntax?

You define a custom provider by specifying an object with a 'provide' key for the token and a 'useValue' key for the fixed value to inject, like { provide: 'TOKEN', useValue: someValue }.

intermediate

Explain the difference between 'useClass' and 'useFactory' in custom providers.

'useClass' tells NestJS to instantiate a specific class when injecting, while 'useFactory' lets you provide a function that returns the value or instance, allowing dynamic or async creation.

intermediate

What is the purpose of the 'inject' array in a custom provider using 'useFactory'?

The 'inject' array lists the dependencies that NestJS should pass as arguments to the factory function, enabling you to use other providers inside your factory.

intermediate

Why would you use a custom provider instead of a regular service class in NestJS?

Custom providers let you control exactly how and what is injected, useful for injecting constants, third-party libraries, or creating dynamic or mocked instances for testing.

Which property do you use to provide a fixed value in a custom provider?

A useFactory

B useClass

C useValue

D useExisting

The 'useValue' property is used to provide a fixed value directly as the provider.

What does the 'useClass' property do in a custom provider?

A Specifies a class to instantiate

B Reuses an existing provider

C Defines a factory function

D Returns a fixed value

'useClass' tells NestJS to instantiate the given class when injecting.

How do you inject dependencies into a factory function in a custom provider?

A Using 'useClass'

B Using the 'inject' array

C Using 'useValue'

D Using 'provide'

The 'inject' array lists dependencies to pass as arguments to the factory function.

Which custom provider type allows asynchronous creation of the injected value?

A useFactory

B useClass

C useValue

D useExisting

'useFactory' can return a Promise, enabling async creation.

What is a common use case for custom providers in NestJS?

A To create UI components

B To style HTML elements

C To write SQL queries

D To control dependency injection behavior

Custom providers let you control how dependencies are created and injected.

Describe how to create a custom provider in NestJS using 'useFactory' and explain when you might use it.

Think about when you need to create a value dynamically or asynchronously.

Explain the differences between 'useValue', 'useClass', and 'useFactory' in NestJS custom providers.

Consider how each method controls what gets injected.