class-validator decorators in NestJS - Step-by-Step Execution

Concept Flow - class-validator decorators

Define Class with Properties

↓

Add Validation Decorators

↓

Create Instance of Class

↓

Call validate() Function

↓

Check Validation Results

↓

Proceed

This flow shows how you define a class with decorators, create an instance, validate it, and handle results.

Execution Sample

NestJS

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

class User {
  @IsString()
  name: string;

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

Defines a User class with validation rules on name and age properties.

Execution Table

Step	Action	Property	Decorator Check	Validation Result
1	Create User instance	name	N/A	N/A
2	Create User instance	age	N/A	N/A
3	Validate 'name'	@IsString()	Check if value is string	Pass if string, fail otherwise
4	Validate 'age'	@IsInt()	Check if value is integer	Pass if integer, fail otherwise
5	Validate 'age'	@Min(0)	Check if value >= 0	Pass if >=0, fail otherwise
6	Collect errors	All properties	Aggregate failed validations	Empty if all pass, list errors if any fail
7	Return validation result	All properties	N/A	Valid or Invalid with errors

💡 Validation ends after checking all decorated properties and collecting errors.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
user.name	undefined	"Alice"	"Alice"	"Alice"	"Alice" (valid)
user.age	undefined	25	25	25	25 (valid)
validationErrors	[]	[]	[]	[]	[] (no errors)

Key Moments - 3 Insights

Why does validation fail if 'age' is a string like '25'?

Because @IsInt() requires a number type, not a string. See execution_table step 4 where the integer check fails.

What happens if 'name' is missing or empty?

If 'name' is undefined or not a string, @IsString() fails validation as shown in execution_table step 3.

Can multiple decorators be used on one property?

Yes, like @IsInt() and @Min(0) on 'age'. Both checks run sequentially (steps 4 and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what validation happens at step 5?

A Check if 'name' is a string

B Check if 'age' is an integer

C Check if 'age' is at least 0

D Collect all validation errors

💡 Hint

Refer to execution_table row with Step 5 describing @Min(0) check.

According to variable_tracker, what is the value of 'user.name' after step 3?

A undefined

B "Alice"

C 25

D []

💡 Hint

Check variable_tracker row for 'user.name' after Step 3.

If 'user.age' was set to -5, which step would fail validation?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look at execution_table step 5 where @Min(0) checks the minimum value.

Concept Snapshot

class-validator decorators add rules to class properties.
Use @IsString(), @IsInt(), @Min() etc. above properties.
Call validate() to check if values follow rules.
Validation returns errors or success.
Multiple decorators can combine checks on one property.

Full Transcript

This visual execution shows how class-validator decorators work in NestJS. First, you define a class with properties and add decorators like @IsString() or @IsInt() to set validation rules. When you create an instance and call validate(), each property is checked step-by-step. For example, 'name' must be a string, and 'age' must be an integer and at least zero. The execution table traces these checks and their results. The variable tracker shows how values change and validation errors accumulate. Key moments clarify common confusions like type mismatches or multiple decorators. The quiz tests understanding of validation steps and variable states. This helps beginners see exactly how validation runs and what happens when rules pass or fail.

Practice

1. What is the main purpose of @IsEmail() decorator in class-validator when used in a NestJS DTO class?

easy

A. To convert the property value to lowercase

B. To check if the property value is a valid email address format

C. To make the property optional

D. To trim whitespace from the property value

Solution

1. Step 1: Understand the decorator's role

   The @IsEmail() decorator validates that the decorated property contains a valid email format string.

2. Step 2: Compare options with decorator behavior

   Only To check if the property value is a valid email address format matches the validation purpose of @IsEmail(). Other options describe unrelated behaviors.

3. Final Answer:

   To check if the property value is a valid email address format -> Option B

4. Quick Check:

   @IsEmail() validates email format [OK]

Hint: Remember: @IsEmail() checks email format only [OK]

Common Mistakes:

• Thinking @IsEmail() modifies the value instead of validating
• Confusing @IsEmail() with @IsOptional() or @Transform decorators

2. Which of the following is the correct way to use @Length() decorator to require a string property to be between 5 and 10 characters in NestJS?

easy

A. @Length(5, 10)

B. @Length(10, 5)

C. @Length(min=5, max=10)

D. @Length({min:5, max:10})

Solution

1. Step 1: Recall the syntax of @Length decorator

   The @Length() decorator takes two numeric arguments: minimum length first, then maximum length.

2. Step 2: Match options with correct syntax

   @Length(5, 10) correctly uses @Length(5, 10). Options A, B, and D use incorrect argument order or syntax.

3. Final Answer:

   @Length(5, 10) -> Option A

4. Quick Check:

   @Length(min, max) = @Length(5, 10) [OK]

Hint: @Length(min, max) uses two numbers, min first [OK]

Common Mistakes:

• Swapping min and max values
• Using object syntax instead of positional arguments
• Using named parameters which are invalid

3. Given this DTO class, what will be the validation result for username = 'abc' and age = 17?

class UserDto {
  @Length(5, 15)
  username: string;

  @Min(18)
  age: number;
}

medium

A. Validation passes with no errors

B. Validation fails because username is too short

C. Validation fails because age is less than 18

D. Validation fails because both username and age are invalid

Solution

1. Step 1: Check username length

   The username 'abc' has length 3, which is less than the minimum 5 required by @Length(5, 15). So username is invalid.

2. Step 2: Check age minimum

   The age is 17, which is less than the minimum 18 required by @Min(18). So age is invalid.

3. Final Answer:

   Validation fails because both username and age are invalid -> Option D

4. Quick Check:

   username too short + age too low = both invalid [OK]

Hint: Check each decorator rule separately for all properties [OK]

Common Mistakes:

• Ignoring one invalid field and assuming validation passes
• Confusing @Min with @Max for age
• Assuming length includes spaces or trimming automatically

4. Identify the error in this DTO class using class-validator decorators:

class ProductDto {
  @IsInt()
  @Min(1)
  price: string;
}

medium

A. price should be a number type, not string

B. Missing @IsString() decorator for price

C. @Min(1) cannot be used with @IsInt()

D. No error, code is correct

Solution

1. Step 1: Analyze property type and decorators

   The property price is declared as string, but decorators @IsInt() and @Min(1) expect a number type.

2. Step 2: Identify mismatch causing validation error

   Since price is string, validation will fail because @IsInt() requires a number. The type should be number.

3. Final Answer:

   price should be a number type, not string -> Option A

4. Quick Check:

   Type mismatch between property and decorators = error [OK]

Hint: Match property type with decorator expectations [OK]

Common Mistakes:

• Using string type with number validators
• Assuming decorators convert types automatically
• Ignoring TypeScript type declarations

5. You want to validate a DTO property tags which is an array of strings, each string must be at least 3 characters. Which combination of class-validator decorators correctly enforces this?

hard

A. @IsString()\n@Length(3, 50)\n@IsArray()

B. @IsArray()\n@IsString()\n@Length(3, 50)

C. @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true })

D. @IsArray()\n@ArrayMaxSize(1)\n@IsString({ each: false })

Solution

1. Step 1: Validate the property is an array

   Use @IsArray() to ensure tags is an array type.

2. Step 2: Validate each element is a string with length constraints

   Use @IsString({ each: true }) and @Length(3, 50, { each: true }) to apply string and length validation to every item in the array.

3. Step 3: Ensure array has at least one element

   @ArrayMinSize(1) ensures the array is not empty.

4. Final Answer:

   @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true }) -> Option C

5. Quick Check:

   Array + each:true for element validation = @IsArray()\n@ArrayMinSize(1)\n@IsString({ each: true })\n@Length(3, 50, { each: true }) [OK]

Hint: Use { each: true } to validate every array element [OK]

Common Mistakes:

• Forgetting { each: true } for array element validation
• Using @IsString() without array context
• Setting wrong array size constraints