How Class Validator works in TypeScript ?

The class-validator is a TypeScript library that is used to validate the classes. It provides different decorators to validate different types of data in a class. It is mostly used with the NestJS and the TypeORM frameworks. You can define different validation rules for different properties of the class using the decorators provided by this library.

Steps to use class-validator library

Step 1: Install class-validator

Use the below npm command to install the class-validator library into your project folder to use it.

npm install class-validator

Step 2: Import required decorators

Now, you can import the decorators you need to validate the class properties from the class-validator library.

import {commaSeparatedDecorators} from 'class-validator';

Step 3: Use the decorators in the class

It is time to use the imported validators in the class to validate the defined properties.

class className {
    @decorator1
    property1: type1;
    @decorator2
    property2: type2;
}

Step 4: Validate the class instance using the validator

Now, you have to validate the class instance by importing the validate() method from the class validator.

import {validate} from 'class-validator';
validate(classinstance).then(()=>{});

Example: The below code example implements the class-validator library in TypeScript.

Javascript

// index.ts file

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

class validatorClass {
    @IsString()
    name: string;
    desc: string;

    @IsInt()
    workForce: number;

    @IsBoolean()
    isGFG: boolean;
}

const validatorObj = new validatorClass();
validatorObj.name = 'GeeksforGeeks';
validatorObj.desc = 'A Computer Science Portal.';
validatorObj.workForce = 200;
validatorObj.isGFG = true;

validate(validatorObj).then((errors) => {
    if (errors.length > 0) {
        console.log('Validation failed: ', errors);
    } else {
        if (validatorObj.isGFG) {
            console.log(
                'Validation done successfully, ',
                'The class object is: ',
                validatorObj
            );
        }
    }
});

Output: