class-validator — Gate Guard Decorators

Now that you know what validation is, it's time to learn about class-validator — a decorator library that turns ordinary DTO classes into real guard posts. Each decorator is like a separate guard specializing in a specific check.

Installation

1npm install class-validator class-transformer

Basic Decorators

@IsString() — the text guard

Checks whether the value is a string. It's like a guard verifying that the document is written on papyrus, not on stone:

1import { IsString } from 'class-validator';
2
3export class CreateLegionaryDto {
4  @IsString()
5  name: string;  // "Marcus" ✅, 12345 ❌
6}

@IsNumber() — the number guard

Verifies whether the value is a number:

1import { IsNumber } from 'class-validator';
2
3export class CreateLegionaryDto {
4  @IsNumber()
5  age: number;  // 25 ✅, "twenty five" ❌
6}

@IsEmail() — the message guard

Checks the validity of an email address format:

1import { IsEmail } from 'class-validator';
2
3export class CreateLegionaryDto {
4  @IsEmail()
5  cursusPublicus: string;  // "marcus@roma.com" ✅, "marcus" ❌
6}

@IsNotEmpty() — the empty field guard

Will not let empty values through — every field must have content:

1import { IsNotEmpty, IsString } from 'class-validator';
2
3export class CreateLegionaryDto {
4  @IsString()
5  @IsNotEmpty()
6  name: string;  // "Marcus" ✅, "" ❌, null ❌
7}

@MinLength() and @MaxLength() — the length guards

Control the minimum and maximum text length:

1import { MinLength, MaxLength, IsString } from 'class-validator';
2
3export class CreateLegionaryDto {
4  @IsString()
5  @MinLength(2)
6  @MaxLength(50)
7  name: string;  // "Marcus" ✅, "M" ❌, "A".repeat(51) ❌
8}

Combining Decorators

The real power of class-validator lies in combining multiple decorators on a single field. It's like several guards checking different aspects of one document:

1import {
2  IsString,
3  IsNotEmpty,
4  MinLength,
5  MaxLength,
6  IsNumber,
7  IsEmail,
8  Min,
9  Max,
10} from 'class-validator';
11
12export class CreateLegionaryDto {
13  @IsString()
14  @IsNotEmpty()
15  @MinLength(2)
16  @MaxLength(50)
17  name: string;
18
19  @IsString()
20  @IsNotEmpty()
21  rank: string;
22
23  @IsNumber()
24  @Min(16)
25  @Max(65)
26  age: number;
27
28  @IsEmail()
29  cursusPublicus: string;
30
31  @IsString()
32  @IsNotEmpty()
33  legio: string;
34}

Custom Error Messages

Each decorator accepts an optional parameter with an error message — so the guard can explain what's wrong:

1export class CreateLegionaryDto {
2  @IsString({ message: 'Name must be a text, legionary!' })
3  @IsNotEmpty({ message: 'Name cannot be empty!' })
4  @MinLength(2, { message: 'Name must have at least 2 characters!' })
5  name: string;
6
7  @IsNumber({}, { message: 'Age must be a number!' })
8  @Min(16, { message: 'You must be at least 16 years old to join the legion!' })
9  age: number;
10}

Thanks to class-validator, you create a strong guard that automatically rejects invalid data before it reaches the business logic. In the next lesson, you will learn advanced decorators!