Custom validation decorators in NestJS - Step-by-Step Execution

Concept Flow - Custom validation decorators

Define validation logic function

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Create decorator using ValidatorConstraint

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Apply decorator to DTO property

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NestJS runs validation on DTO

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Custom validator checks value

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Allow request

This flow shows how you define a custom validator, create a decorator, apply it to data, and how NestJS uses it to accept or reject input.

Execution Sample

NestJS

import { registerDecorator, ValidationOptions, ValidatorConstraint, ValidatorConstraintInterface } from 'class-validator';

@ValidatorConstraint({ async: false })
class IsEvenConstraint implements ValidatorConstraintInterface {
  validate(value: number) { return value % 2 === 0; }
}

export function IsEven(validationOptions?: ValidationOptions) {
  return function (object: Object, propertyName: string) {
    registerDecorator({
      target: object.constructor,
      propertyName: propertyName,
      options: validationOptions,
      constraints: [],
      validator: IsEvenConstraint,
    });
  };
}

Defines a custom decorator @IsEven that checks if a number is even.

Execution Table

Step	Action	Input Value	Validation Result	Effect
1	Create IsEvenConstraint class	-	-	Validator logic ready
2	Define @IsEven decorator	-	-	Decorator function ready
3	Apply @IsEven to DTO property 'age'	-	-	Property marked for validation
4	Validate DTO with age=4	4	true	Passes validation
5	Validate DTO with age=5	5	false	Fails validation, error returned
6	Validate DTO with age=0	0	true	Passes validation
7	Validate DTO with age=-2	-2	true	Passes validation
8	Validate DTO with age=3.5	3.5	false	Fails validation, error returned
9	Validation ends	-	-	Request accepted or rejected based on result

💡 Validation stops after checking the input value with the custom validator logic.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
value	-	4	5	0	-2	3.5	-
validationResult	-	true	false	true	true	false	-

Key Moments - 3 Insights

Why does the validation fail for 3.5 even though it's a number?

The custom validator checks if the number is even using modulo (% 2 === 0). Since 3.5 % 2 is not zero, it fails. See execution_table row 8.

What happens if we forget to register the decorator with registerDecorator?

The validation will not run because NestJS won't know about the custom validator. The decorator must call registerDecorator as shown in execution_table step 2.

Can the validator handle negative even numbers?

Yes, negative even numbers like -2 pass because modulo works the same. See execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 5. What is the validation result for input value 5?

A true

B false

C undefined

D error

💡 Hint

Check the 'Validation Result' column at step 5 in the execution_table.

At which step does the validation pass for the input value 0?

A Step 4

B Step 6

C Step 7

D Step 8

💡 Hint

Look for input value 0 in the 'Input Value' column and see its validation result.

If the validate method returned true for all numbers, how would the validation results change?

A All inputs would pass validation

B Only even numbers would pass

C Only odd numbers would pass

D Validation would always fail

💡 Hint

Consider what happens if validate always returns true regardless of input.

Concept Snapshot

Custom validation decorators in NestJS:
- Create a class implementing ValidatorConstraintInterface
- Define validate() with your logic
- Use @ValidatorConstraint() on the class
- Create a decorator function calling registerDecorator
- Apply decorator to DTO properties
- NestJS runs your validator during validation pipeline

Full Transcript

This example shows how to create a custom validation decorator in NestJS using class-validator. First, you define a class with a validate method that checks if a number is even. Then, you decorate this class with @ValidatorConstraint. Next, you create a decorator function that registers this validator with registerDecorator. When you apply this decorator to a DTO property, NestJS will run your custom validation logic during request validation. The execution table traces validation for different input values, showing which pass or fail. Key moments clarify common confusions like handling decimals or negative numbers. The visual quiz tests understanding of validation results at specific steps. This approach helps you add your own rules beyond built-in validators in NestJS.