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

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  • Last Updated : 24 Dec, 2021
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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 Ascii characters are allowed i.e. there is not allowed any Non-ASCII characters (Ex: ñ). We can also validate these input fields to accept only ASCII 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 isAscii() 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 accept only ascii characters.

Filename – index.js


const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateAsciiCharacters } = 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 to submit ascii text
app.get('/', (req, res) => {
// Post route to handle form submission logic and
  async (req, res) => {
    const errors = validationResult(req)
      return res.send(formTemplet({errors}))
    const {submittedBy, ascii} = req.body
    await repo.create({submittedBy, ascii})
    res.send('Ascii Characters registered successfully')
// 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) {
    // The filename where datas 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){
    const records = await this.getAll()
    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 to submit Ascii text.


const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
module.exports = ({errors}) => {
  return `
    <!DOCTYPE html>
        <link rel='stylesheet'
            margin-top: 100px;
            margin-top : 10px
        <div class='container'>
          <div class='columns is-centered'>
            <div class='column is-5'>
              <form action='/register/ascii' method='POST'>            
                    <label class='label' id='submittedBy'>
                      Submitted By
                  <input class='input' type='text'
                    <label class='label' id='ascii'>
                      Ascii Characters
                  <input class='input' type='text'
                         name='ascii' placeholder='Ascii
                         characters' for='ascii'>
                  <p class="help is-danger">
                    ${getError(errors, 'ascii')}
                  <button class='button is-primary'>

Filename – validator.js: This file contain all the validation logic(Logic to validate a input field to accept only ascii characters).


const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateAsciiCharacters : check('ascii')
    // To delete leading and trailing space
    // Validate minimum length of ascii text
    // Optional for this context
'Please submit minimum 8 characters long Ascii text')
    // Validate input field to accept only ascii chars
    .withMessage('Must be Ascii characters only')

Filename – package.json

package.json file




Attempt to submit form when Ascii characters input field contains non ascii characters

Response when attempt to submit form where Ascii characters input field contains non ascii characters

Attempt to submit form when Ascii characters input field contains only ascii characters

Response when attempt to submit form where Ascii characters input field contains only ascii characters

Database after successful form submission:

Database after successful form submission

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

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