How to validate if input in input field has alphanumeric characters only using express-validator ?

27 Dec, 2021
In HTML forms, we often required validation of different types. Validate existing email, validate password length, validate confirm password, validate to allow only integer inputs, these are some examples of validation. In a certain input field, only alphanumeric characters are allowed i.e. there not allowed any special characters. We can also validate these input fields to accept only alphanumeric characters using express-validator middleware.
Command to install express-validator:
npm install express-validator
Steps to use express-validator to implement the logic:
Install express-validator middleware.
Create a validator.js file to code all the validation logic.
Validate input by validateInputField: check(input field name) and chain on the validation isAlphanumeric() with ‘ . ‘
Use the validation name(validateInputField) in the routes as a middleware as an array of validations.
Destructure ‘validationResult’ function from express-validator to use it to find any errors.
If error occurs redirect to the same page passing the error information.
If error list is empty, give access to the user for the subsequent request.
Note: Here we use local or custom database to implement the logic, the same steps can be followed to implement the logic in a regular database like MongoDB or MySql.
Example: This example illustrates how to validate a input field to only allow the alphabets.
Filename – index.js
const express = require('express')const bodyParser = require('body-parser')const {validationResult} = require('express-validator')const repo = require('./repository')const { validateUsername } = require('./validator')const signupTemplet = require('./signup')  const app = express()const port = process.env.PORT || 3000  // The body-parser middleware to parse form dataapp.use(bodyParser.urlencoded({extended : true}))  // Get route to display HTML form to sign upapp.get('/signup', (req, res) => {  res.send(signupTemplet({}))})  // Post route to handle form submission logic and app.post(  '/signup',  [validateUsername],  async (req, res) => {    const errors = validationResult(req)    if(!errors.isEmpty()) {      return res.send(signupTemplet({errors}))    }    const {email, username, password} = req.body    await repo.create({      email,       username,      password    })    res.send('Sign Up successfully')})  // Server setupapp.listen(port, () => {  console.log(`Server start on port ${port}`)})
Filename – repository.js: This file contains all the logic to create a local database and interact with it.
// Importing node.js file system module const fs = require('fs')  class Repository {  constructor(filename) {        // Filename where datas are going to store    if(!filename) {      throw new Error('Filename is required to create a datastore!')    }      this.filename = filename      try {      fs.accessSync(this.filename)    } catch(err) {        // If file not exist it is created with empty array      fs.writeFileSync(this.filename, '[]')    }  }    // Get all existing records  async getAll(){    return JSON.parse(      await fs.promises.readFile(this.filename, {        encoding : 'utf8'      })    )  }    // Create new record  async create(attrs){    const records = await this.getAll()    records.push(attrs)    await fs.promises.writeFile(      this.filename,      JSON.stringify(records, null, 2)       )    return attrs  }}  // The 'datastore.json' file created at runtime // and all the information provided via signup form// store in this file in JSON format.module.exports = new Repository('datastore.json')
Filename – signup.js: This file contains logic to show sign up form.
const getError = (errors, prop) => {  try {    return errors.mapped()[prop].msg  } catch (error) {    return ''  } }  module.exports = ({errors}) => {  return `    <!DOCTYPE html>    <html>      <head>        <link rel='stylesheet' href='https://cdnjs.cloudflare.com/ajax/libs/bulma/0.9.0/css/bulma.min.css'>        <style>          div.columns{            margin-top: 100px;          }          .button{            margin-top : 10px          }        </style>      </head>      <body>        <div class='container'>          <div class='columns is-centered'>            <div class='column is-5'>              <h1 class='title'>Sign Up<h1>              <form method='POST'>                             <div>                  <div>                    <label class='label' id='email'>Email</label>                  </div>                  <input class='input' type='text' name='email'                   placeholder='Email' for='email'>                </div>                <div>                  <div>                    <label class='label' id='fn'>Username</label>                  </div>                  <input class='input' type='text' name='username'                  placeholder='Username' for='username'>                  <p class="help is-danger">${getError(errors, 'username')}</p>                </div>                <div>                  <div>                    <label class='label' id='password'>Password</label>                  </div>                  <input class='input' type='password' name='password'                   placeholder='Password' for='password'>                </div>                <div>                  <button class='button is-primary'>Sign Up</button>                </div>              </form>            </div>          </div>        </div>      </body>    </html>     `}
Filename – validator.js: This file contain all the validation logic(Logic to validate a input field to only allow the alphanumeric characters).
const {check} = require('express-validator')const repo = require('./repository')module.exports = {  validateUsername : check('username')      // To delete leading and trailing space    .trim()      // Validate minimum length of password    // Optional for this context    .isLength({min:4})      // Custom message    .withMessage('Username must be minimum 4 characters long')      // Validate username to be alphanumeric    .isAlphanumeric()      // Custom message    .withMessage('Username must be alphanumeric')  }
Filename – package.json
package.json file
Database:
Database
Output:
Attempt to sign up when username input field is not alphanumeric
Response when attempt to sign up with username input field which is not alphanumeric
Attempt to sign up when username input field contains only alphanumeric character
Response when attempt to sign up with username input field which contains only alphanumeric characters
Database after successful Sign Up:
Database after successful Sign Up
Note: We have used some Bulma classes(CSS framework) in the signup.js file to design the content.
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