body("thumbnail_file").custom(async (value, { req }) => {

  const thumbnailFile = req.files && req.files.thumbnail_file;

  if (!thumbnailFile) {

    return true;

  } else if (Array.isArray(thumbnailFile)) {

    throw new Error("Only one thumbnail file is allowed.");

  }

  if (

    thumbnailFile.mimetype !== "image/png" &&

    thumbnailFile.mimetype !== "image/jpg" &&

    thumbnailFile.mimetype !== "image/jpeg"

  ) {

    throw new Error(

      "Only .png, .jpg and .jpeg image formats are allowed for thumbnail file."

    );

  }

}),

npm install --save express-validator

​

npm install --save express-validator

// ...rest of the initial code omitted for simplicity.

const { body, validationResult } = require('express-validator');

​

app.post(

  '/user',

  // username must be an email

  body('username').isEmail(),

  // password must be at least 5 chars long

  body('password').isLength({ min: 5 }),

  (req, res) => {

    // Finds the validation errors in this request and wraps them in an object with handy functions

    const errors = validationResult(req);

    if (!errors.isEmpty()) {

      return res.status(400).json({ errors: errors.array() });

    }

​

    User.create({

      username: req.body.username,

      password: req.body.password,

    }).then(user => res.json(user));

  },

);

const { checkSchema, validationResult } = require('express-validator');

​

const validationSchema = checkSchema({

  password: {

    isLength: {

      options: { min: 5 },

      errorMessage: 'Password must be at least 5 characters long',

    },

    // Custom validation: check if password confirmation matches the password

    custom: {

      options: (value, { req }) => {

        if (value !== req.body.confirmPassword) {

          throw new Error('Password confirmation does not match password');

        }

        return true;

      },

    },

  },

  age: {

    // Custom validation: check if the age is an even number

    custom: {

      options: (value) => {

        if (value % 2 !== 0) {

          throw new Error('Age must be an even number');

        }

        return true;

      },

    },

  },

});

​

app.post('/validate', validationSchema, (req, res) => {

  const errors = validationResult(req);

  if (!errors.isEmpty()) {

    return res.status(400).json({ errors: errors.array() });

  }

  res.send('Validation passed!');

});

​