class-transformer usage in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the class-transformer library in NestJS?

It helps convert plain JavaScript objects into class instances and vice versa, making it easier to work with typed data and apply transformations or validations.

beginner

How do you use @Expose() decorator in class-transformer?

The <code>@Expose()</code> decorator marks a class property to be included when transforming an object to plain or class instance, allowing control over which properties are visible.

beginner

What does the plainToInstance() function do?

It converts a plain JavaScript object into an instance of a specified class, applying any decorators like @Type() or @Expose() during the transformation.

intermediate

Why would you use @Type(() => SomeClass) in a DTO class?

To tell class-transformer how to transform nested objects or arrays into instances of the specified class, ensuring proper typing and transformation of nested data.

intermediate

What is the effect of using excludeExtraneousValues: true option in transformation?

It removes any properties from the plain object that do not have @Expose() decorator in the class, helping to keep only intended data and improve security.

Which function converts a plain object to a class instance in class-transformer?

A plainToInstance()

B classToPlain()

C transformObject()

D toClass()

The plainToInstance() function converts plain objects to class instances.

What does the @Expose() decorator do?

A Validates property values

B Hides properties from transformation

C Marks properties to be included during transformation

D Converts property types automatically

@Expose() marks properties to be included when transforming objects.

How do you specify the type of a nested object for transformation?

A @ExposeNested(ClassName)

B @NestedType(ClassName)

C @TransformType(ClassName)

D @Type(() => ClassName)

Use @Type(() => ClassName) to specify nested object types.

What happens if you set excludeExtraneousValues: true during transformation?

A Only properties with @Expose() are included

B All properties are included

C Transformation fails if extra properties exist

D Properties are renamed automatically

Only properties decorated with @Expose() are included when excludeExtraneousValues is true.

Which decorator is NOT part of class-transformer?

A @Expose()

B @Validate()

C @Type()

D @Exclude()

@Validate() is from class-validator, not class-transformer.

Explain how you would use class-transformer to convert a plain JSON object into a typed class instance with nested objects in NestJS.

Think about how to prepare your classes and call the right function.

Describe the benefits of using excludeExtraneousValues: true option in data transformation.

Consider what happens to properties without @Expose() when this option is enabled.