DTO class creation in NestJS - Performance & Optimization

Performance: DTO class creation

LOW IMPACT

This affects the bundle size and initial load time by adding extra code for data validation and transformation.

Defining data transfer objects (DTOs) for API input validation

NestJS

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

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

  @IsInt()
  @Min(0)
  @Max(120)
  age: number;
}

Removing unnecessary fields and validations reduces bundle size and speeds up parsing.

📈 Performance Gainsaves ~5kb bundle size, reduces blocking time by 10ms

Defining data transfer objects (DTOs) for API input validation

NestJS

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

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

  @IsInt()
  @Min(0)
  @Max(120)
  age: number;

  @IsString()
  bio: string;

  @IsString()
  extraLargeField: string; // very large string field not always needed
}

Including large or unnecessary fields and complex validations increases bundle size and slows initial load.

📉 Performance Costadds ~5kb to bundle, blocks rendering for 10-15ms on slow devices

Performance Comparison

Pattern	Bundle Size Impact	Execution Time	Validation Complexity	Verdict
Large DTO with many validations and fields	High (~5kb+)	High (10-15ms blocking)	Complex	[X] Bad
Minimal DTO with essential validations only	Low (~1-2kb)	Low (2-5ms blocking)	Simple	[OK] Good

Rendering Pipeline

DTO classes are parsed and validated during request handling, affecting JavaScript execution and initial bundle parsing.

→JavaScript Parsing

→Script Execution

⚠️ BottleneckScript Execution due to validation decorators and metadata processing

Core Web Vital Affected

LCP

This affects the bundle size and initial load time by adding extra code for data validation and transformation.

Optimization Tips

1Keep DTO classes small and focused to reduce bundle size.

2Avoid unnecessary validation decorators to minimize script execution time.

3Use DTOs only for required data fields to improve load speed.

Performance Quiz - 3 Questions

How does adding many validation decorators to a DTO class affect performance?

A Decreases bundle size but increases execution time

B Increases bundle size and script execution time

C Has no effect on performance

D Only affects CSS rendering

DevTools: Performance

How to check: Record a performance profile during app startup or API request handling; look for long script execution times related to DTO validation.

What to look for: Long scripting tasks or blocking time caused by class-validator decorators or metadata reflection.

Practice

1. What is the main purpose of a DTO class in a NestJS application?

easy

A. To handle database connections and queries

B. To create UI components for the frontend

C. To define the shape and validation rules of data transferred between layers

D. To manage application routing and middleware

Solution

1. Step 1: Understand the role of DTOs

   DTO stands for Data Transfer Object, used to define how data should look when sent or received.

2. Step 2: Identify what DTOs do in NestJS

   In NestJS, DTOs specify the structure and validation of data between client and server or between app layers.

3. Final Answer:

   To define the shape and validation rules of data transferred between layers -> Option C

4. Quick Check:

   DTO = Data shape and validation [OK]

Hint: DTOs organize and validate data structure in NestJS [OK]

Common Mistakes:

• Confusing DTOs with database models
• Thinking DTOs handle routing or UI
• Assuming DTOs manage middleware

2. Which of the following is the correct way to declare a required string property name in a NestJS DTO class using class-validator decorators?

easy

A. @IsString() name: string;

B. name: string;

C. @IsOptional() name: string;

D. @IsNumber() name: string;

Solution

1. Step 1: Identify the decorator for string validation

   The @IsString() decorator from class-validator ensures the property is a string and required by default.

2. Step 2: Check other options

   name: string; lacks validation, C marks it optional, and D wrongly uses number validation.

3. Final Answer:

   @IsString() name: string; -> Option A

4. Quick Check:

   Use @IsString() for required string fields [OK]

Hint: Use @IsString() to require string fields in DTOs [OK]

Common Mistakes:

• Omitting validation decorators
• Using @IsOptional() for required fields
• Applying wrong type decorators like @IsNumber()

3. Given this DTO class, what will be the result of validating an object { name: "Alice", age: 25 }?

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

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

  @IsInt()
  @IsOptional()
  age?: number;
}

medium

A. Validation passes because name is string and age is optional integer

B. Validation fails because age is required but missing

C. Validation fails because name is not optional

D. Validation passes only if age is a string

Solution

1. Step 1: Analyze the DTO decorators

   Name is required string, age is optional integer due to @IsOptional() and @IsInt().

2. Step 2: Check the input object

   Input has name as string "Alice" and age as number 25, matching the DTO rules.

3. Final Answer:

   Validation passes because name is string and age is optional integer -> Option A

4. Quick Check:

   Required string + optional int with valid data = pass [OK]

Hint: Optional fields pass validation if present and valid [OK]

Common Mistakes:

• Assuming optional fields must be present
• Confusing @IsOptional() effect
• Thinking validation fails if optional field is present

4. Identify the error in this DTO class that causes validation to always fail for the email property:

import { IsEmail } from 'class-validator';

export class EmailDto {
  @IsEmail
  email: string;
}

medium

A. Should import IsString instead of IsEmail

B. email property should be optional

C. email property type should be number

D. Missing parentheses after @IsEmail decorator

Solution

1. Step 1: Check decorator usage

   @IsEmail is a function decorator and must be called with parentheses: @IsEmail()

2. Step 2: Understand effect of missing parentheses

   Without parentheses, the decorator is not applied properly, causing validation to fail.

3. Final Answer:

   Missing parentheses after @IsEmail decorator -> Option D

4. Quick Check:

   Decorators need () even if no args [OK]

Hint: Always add () after decorators like @IsEmail() [OK]

Common Mistakes:

• Forgetting parentheses on decorators
• Changing property type incorrectly
• Using wrong decorators for email

5. You want to create a DTO class CreateProductDto with these rules:
- title is a required string
- price is a required positive number
- description is an optional string
Which of the following DTO class definitions correctly implements these rules?

hard

A. import { IsString, IsNumber, IsPositive, IsOptional } from 'class-validator';

export class CreateProductDto {
@IsString()
title?: string;

@IsNumber()
@IsPositive()
price?: number;

@IsOptional()
@IsString()
description?: string;
}

B. import { IsString, IsNumber, IsPositive, IsOptional } from 'class-validator';

export class CreateProductDto {
@IsString()
title: string;

@IsNumber()
@IsPositive()
price: number;

@IsOptional()
@IsString()
description?: string;
}

C. import { IsString, IsNumber, IsPositive } from 'class-validator';

export class CreateProductDto {
@IsString()
title: string;

@IsNumber()
price: number;

@IsString()
description: string;
}

D. import { IsString, IsNumber, IsPositive, IsOptional } from 'class-validator';

export class CreateProductDto {
@IsString()
title: string;

@IsNumber()
price: number;

@IsOptional()
description?: string;
}

Solution

1. Step 1: Check required string property

   Title is required string, so @IsString() with no @IsOptional() is correct.

2. Step 2: Check required positive number property

   Price must be number and positive, so @IsNumber() and @IsPositive() are needed without @IsOptional().

3. Step 3: Check optional string property

   Description is optional string, so @IsOptional() and @IsString() with optional property syntax (?) is correct.

4. Final Answer:

   DTO with @IsString() title, @IsNumber() + @IsPositive() price, and optional @IsOptional() description -> Option B

5. Quick Check:

   Required fields no @IsOptional(), optional fields with @IsOptional() [OK]

Hint: Use @IsOptional() only on optional fields [OK]

Common Mistakes:

• Marking required fields as optional
• Missing @IsPositive() for positive number
• Omitting @IsOptional() on optional fields