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

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Check Validation Results

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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.