validation-final-index.ts

import { Router, Response, Request, NextFunction } from "express";
import express from "express";
import bodyParser from "body-parser";
import { validate, Matches, IsDefined } from "class-validator";
import { plainToClass, Expose } from "class-transformer";

class UserDto {
    @IsDefined()
    @Expose()
    username: String;
    @IsDefined()
    @Expose()
    @Matches(RegExp(/^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[0-9a-zA-Z]{8,}$/))
    password: String;
    @IsDefined()
    @Expose()
    @Matches(RegExp(/^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/))
    email: String;
}

const app = express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));
const router = Router();

const validationMw = (dtoClass: any) => {
    return function (req: Request, res: Response, next: NextFunction) {
        const output: any = plainToClass(dtoClass, req.body);
        validate(output, { skipMissingProperties: true }).then(errors => {
            // errors is an array of validation errors
            if (errors.length > 0) {
                console.log(errors);
                let errorTexts = Array();
                for (const errorItem of errors) {
                    errorTexts = errorTexts.concat(errorItem.constraints);
                }
                res.status(400).send(errorTexts);
                return;
            } else {
                res.locals.input = output;
                next();
            }
        });
    };
};

router.post("/user/submit", validationMw(UserDto), async (req: Request, res: Response) => {
    res.json(res.locals.input);
});

app.use(router);
app.listen(3000, () => {
    console.log('Server is running in http://localhost:3000');
});

Loved your article where this snippet was used!

I have a different way of writing the part of the middleware where you collect any validation errors and send them to the user. If others want to try it out they can feel free. I swapped these lines:

                let errorTexts = Array();
                for (const errorItem of errors) {
                    errorTexts = errorTexts.concat(errorItem.constraints);
                }

for this one

                const errorTexts = errors.map(obj => Object.values(obj.constraints)).flat().join('. ');

This gives all the errors as sentences, one after another, and no brackets or other syntax, so the response is human readable.

Cheers!