Validation with Joi in NestJS

Introduction

Validation with Joi helps check if data is correct before using it. It stops mistakes early and keeps your app safe.

When you want to check user input in forms before saving it.

When receiving data from APIs and you need to ensure it has the right shape.

When you want to give clear error messages if data is missing or wrong.

When you want to avoid crashes caused by bad data.

When you want to keep your app data consistent and clean.

Syntax

NestJS

import * as Joi from 'joi';

const schema = Joi.object({
  name: Joi.string().required(),
  age: Joi.number().integer().min(0),
});

const result = schema.validate(data);

Use Joi.object() to define the shape of your data.

Use .validate() to check data against the schema.

Examples

This schema checks that email is a required string and a valid email format.

NestJS

const schema = Joi.object({
  email: Joi.string().email().required(),
});

This schema requires a password string with at least 8 characters.

NestJS

const schema = Joi.object({
  password: Joi.string().min(8).required(),
});

This schema expects tags to be an array of strings.

NestJS

const schema = Joi.object({
  tags: Joi.array().items(Joi.string()),
});

Sample Program

This NestJS controller uses Joi to check the data sent to the /users POST route. If the data is wrong, it sends an error. If it is correct, it returns a success message with the user data.

NestJS

import { Controller, Post, Body, BadRequestException } from '@nestjs/common';
import * as Joi from 'joi';

const createUserSchema = Joi.object({
  username: Joi.string().alphanum().min(3).max(30).required(),
  email: Joi.string().email().required(),
  age: Joi.number().integer().min(0).optional(),
});

@Controller('users')
export class UsersController {
  @Post()
  createUser(@Body() body: any) {
    const { error, value } = createUserSchema.validate(body);
    if (error) {
      throw new BadRequestException(error.details[0].message);
    }
    return { message: 'User created', user: value };
  }
}

Important Notes

Always validate data before using it to avoid bugs and security issues.

Joi gives detailed error messages to help fix input problems.

You can combine Joi with NestJS pipes for automatic validation.

Summary

Joi helps check data shapes and rules simply.

Use Joi schemas to describe what data should look like.

Validate data early to keep your app safe and stable.