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How to validate if input in input field has hexadecimal number only using express-validator ?

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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 hexadecimal numbers are allowed. We can also validate these input fields to only accept full-width strings using express-validator middleware.

Hexadecimal uses sixteen as a base. Since most computers consist of elements that can only have two stable states, (such as ON or OFF), we use binary (base two) to represent the bits (binary digits) of a set of states in the computer. However, humans, who are more comfortable with number of bases that are somewhat bigger than two. Therefore, it was found to be more convenient to write numbers representing computer memory with fewer digits where each digit comes from a larger set (such as {0123456789ABCDEF} for “Hexadecimal”, which uses base-16).

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 validation isHexadecimal() 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 an input field to only allow hexadecimal numbers.

Filename – index.js


const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateIdentificationNumber } = require('./validator')
const formTemplet = require('./form')
const app = express()
const port = process.env.PORT || 3000
// The body-parser middleware to parse form data
app.use(bodyParser.urlencoded({extended : true}))
// Get route to display HTML form
app.get('/', (req, res) => {
// Post route to handle form submission logic and
  async (req, res) => {
    const errors = validationResult(req)
    if(!errors.isEmpty()) {
      return res.send(formTemplet({errors}))
    const {name, idn} = req.body
    // New record
    await repo.create({
      'Identification Number':idn
    res.send('<strong>Registered successfully..!!</strong>')
// Server setup
app.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 data are going to store
    if(!filename) {
      throw new Error(
'Filename is required to create a datastore!')
    this.filename = filename
    try {
    } 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) {
    // Fetch all existing records
    const records = await this.getAll()
    // All the existing records with new
    // record push back to database
    await fs.promises.writeFile(
      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 – form.js: This file contains logic to show the form.


const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
module.exports = ({errors}) => {
  return `
<!DOCTYPE html>
  <link rel='stylesheet' href=
    div.columns {
      margin-top: 100px;
    .button {
      margin-top: 10px
  <div class='container'>
    <div class='columns is-centered'>
      <div class='column is-5'>
        <form action='/info' method='POST'>
              <label class='label' id='name'>
            <input class='input' type='text'
              name='name' placeholder='Vinit singh'
              <label class='label' id='idn'>
                Hexadecimal Identification Number
            <input class='input' type='text' name='idn'
              'Choose your hexadecimal Identification Number'
            <p class="help is-danger">
              ${getError(errors, 'idn')}
            <button class='button is-primary'>Submit</button>

Filename – validator.js: This file contain all the validation logic(Logic to validate a input field to only allow hexadecimal numbers).


const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateIdentificationNumber : check('idn')
    // To delete leading and trailing space
    // Validate identification number to
    // accept only hexadecimal number
    // Custom message
    .withMessage('Must be a hexadecimal number')  

Filename – package.json

package.json file




Attempt to submit form data when identification number input field is not a valid hexadecimal number

Response when attempt to submit form data where identification number input field is  not a valid hexadecimal number

Attempt to submit form data when identification number input field is  a valid hexadecimal number

Response when attempt to submit form data where identification number input field is a valid hexadecimal number

Database after successful form submission:

Database after successful submission of form

Note: We have used some Bulma classes(CSS framework) in the form.js file to design the content.

Last Updated : 07 Apr, 2022
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