Transforming Payloads
With transform enabled, ValidationPipe turns plain JSON into typed DTO instances and converts primitives.
Syntax
@Type(() => Number)By default, request data arrives as plain objects and strings. With transform: true, the ValidationPipe uses class-transformer to convert payloads into real instances of your DTO classes.
What transformation does
- Creates an actual
CreateUserDtoinstance, so its methods are available. - Converts route and query params to their declared types (e.g. string to number).
- Applies
@Type()hints for nested objects and arrays.
Use @Type(() => Number) from class-transformer when you need explicit conversion of a value.
Example
import { Type } from 'class-transformer';
import { IsInt, Min } from 'class-validator';
export class PaginationDto {
@Type(() => Number) // convert the query string to a number
@IsInt()
@Min(1)
page: number;
}When to use it
- A developer enables transform:true so a route parameter :id received as a string "42" is automatically cast to a number before the handler runs.
- A team uses @Type(() => Number) on a DTO property so JSON strings from form submissions are coerced to numbers during deserialization.
- An engineer uses class-transformer's @Exclude() to strip sensitive fields (like password hashes) from response objects before serialization.
More examples
Auto-transform query to number
Uses @Type(() => Number) to coerce query string values to integers, enabling typed query DTOs with default values.
import { IsInt, Min } from 'class-validator';
import { Type } from 'class-transformer';
export class PaginationDto {
@Type(() => Number) // '10' → 10
@IsInt()
@Min(1)
page: number = 1;
@Type(() => Number)
@IsInt()
@Min(1)
limit: number = 20;
}Exclude sensitive property
Marks a field with @Exclude so class-transformer omits it when the object is serialized in the response.
import { Exclude } from 'class-transformer';
export class UserResponseDto {
id: number;
name: string;
email: string;
@Exclude() // never serialized in the response
passwordHash: string;
constructor(partial: Partial<UserResponseDto>) {
Object.assign(this, partial);
}
}ClassSerializerInterceptor + @Expose
Applies ClassSerializerInterceptor so @Exclude and @Expose decorators on UserResponseDto are honoured automatically.
import { Controller, Get, UseInterceptors, ClassSerializerInterceptor } from '@nestjs/common';
@Controller('users')
@UseInterceptors(ClassSerializerInterceptor)
export class UsersController {
@Get(':id')
findOne(@Param('id', ParseIntPipe) id: number): UserResponseDto {
return new UserResponseDto({ id, name: 'Alice', passwordHash: 'secret' });
}
}
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