DTO class creation in NestJS - Step-by-Step Execution

Concept Flow - DTO class creation

Define DTO class

↓

Add properties with types

↓

Use decorators for validation

↓

Export DTO class

↓

Use DTO in controller/service

This flow shows how to create a DTO class by defining it, adding typed properties, applying validation decorators, exporting it, and then using it in your NestJS app.

Execution Sample

NestJS

import { IsString, IsInt } from 'class-validator';

export class CreateUserDto {
  @IsString()
  name: string;

  @IsInt()
  age: number;
}

Defines a DTO class with two properties, name and age, each with validation decorators.

Execution Table

Step	Action	Code Line	Effect
1	Import validation decorators	import { IsString, IsInt } from 'class-validator';	Decorators available for use
2	Define DTO class	export class CreateUserDto {	Class CreateUserDto created
3	Add name property with @IsString	@IsString() name: string;	Property 'name' expects a string and is validated
4	Add age property with @IsInt	@IsInt() age: number;	Property 'age' expects an integer and is validated
5	Close the DTO class	}	DTO class ready for import and use
6	Use DTO in controller/service	e.g. create(@Body() createUserDto: CreateUserDto)	DTO validates incoming data automatically

💡 DTO class fully defined and ready to validate data in NestJS

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
CreateUserDto	undefined	Class with 'name' property typed string	Class with 'name' and 'age' properties typed	Complete DTO class with validation decorators

Key Moments - 2 Insights

Why do we use decorators like @IsString() on properties?

Decorators tell NestJS how to validate each property. See execution_table step 3 and 4 where @IsString and @IsInt are applied to properties.

Can we use DTO without exporting the class?

No, the 'export' keyword (step 2) makes it available to other files like controllers. Without export, NestJS can't use it.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 3?

A Add a property 'name' with string type and validation

B Import validation decorators

C Export the DTO class

D Use DTO in controller

💡 Hint

Check the 'Action' and 'Effect' columns at step 3 in execution_table

At which step is the DTO class made available for use in other files?

A Step 5

B Step 4

C Step 2

D Step 6

💡 Hint

Check the code line for step 2 in execution_table

If we remove @IsInt() decorator from 'age', what changes in the execution?

A 'age' becomes a string

B No validation on 'age' property

C DTO class won't compile

D 'name' property loses validation

💡 Hint

Refer to step 4 where @IsInt() applies validation to 'age'

Concept Snapshot

DTO class creation in NestJS:
- Define a class with export
- Add typed properties
- Use class-validator decorators (@IsString, @IsInt)
- Import and use DTO in controllers/services
- Validates incoming data automatically

Full Transcript

To create a DTO class in NestJS, first import validation decorators from 'class-validator'. Then define and export a class with typed properties. Apply decorators like @IsString or @IsInt to each property to specify validation rules. This class can then be imported and used in controllers or services to validate incoming data automatically. The execution steps show importing decorators, defining the class, adding properties with decorators, exporting the class, and using it in the app.