Open In App
Related Articles

How to validate if input in input field has full width string only using express-validator ?

Like Article
Save Article
Report issue

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 full-width strings are allowed. We can also validate these input fields to only accept full-width strings 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 validation isFullWidth() 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 the full width strings.

Filename – index.js


const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateMessage } = 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, msg} = req.body
    // New record
    await repo.create({
'<strong>Message send to admin 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 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) {
    // 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='msg'>
                Message to admin
            <input class='input' type='text' name='msg'
              placeholder='Message in full width string'
            <p class="help is-danger">
              ${getError(errors, 'msg')}
            <button class='button is-primary'>


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


const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateMessage : check('msg')
    // To delete leading and trailing space
    // Validate message to accept only
    // the full width string
    // Custom message
    .withMessage('Must be a full width string')  


Online converting normal string to full width string:

online converting normal string to Full width string

Filename – package.json

package.json file




Attempt to submit form data when message input field not contains full width string

Response when attempt to submit form data where message input field not contains full width string

Attempt to submit form data when message input field contains full width string

Response when attempt to submit form data where message input field contains full width string

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 : 08 Apr, 2022
Like Article
Save Article
Share your thoughts in the comments
Similar Reads