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How to validate if input in input field exactly equals to some other value 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 certain cases, we want the user to type some exact value and based on that we give the user access to the request or deny the request access. We can also validate these input fields to accept some exact required values otherwise deny the request 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 equals() with ‘ . ‘
  • We can also use custom validator to validate if want to pass the required text to type in input field as a request body.
  • 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 some exact value.

Filename – index.js


const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateDeclaration } = require('./validator')
const showTemplet = require('./show')
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 type declaration
// and submit to delete the record
app.get('/', (req, res) => {
// Post route to handle form submission logic and
  async (req, res) => {
    const errors = validationResult(req)
      return res.send(showTemplet({errors}))
    const id =
    await repo.delete(id)
    res.send('Record Deleted 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) {
    // Filename where datas are going to store
      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'
  // Delete record
  async delete(id) {
    const records = await this.getAll()
    const filteredRecords = records.filter(record => !== id)
    await fs.promises.writeFile(
      JSON.stringify(filteredRecords, null, 2)  
// 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 – show.js: This file contains logic to show declaration and input field to type declaration.


const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
module.exports = ({errors}) => {
  const id = 'd8f98678eb8a'
  const declaration=
`I read all FAQ and wants to delete the record with id ${id}`
  return `
    <!DOCTYPE html>
        <link rel='stylesheet'
            margin-top: 100px;
            margin-top : 10px
        <div class='container'>
          <div class='columns is-centered'>
            <div class='column is-5'>
              <div class='title is-5'>Declaration :</div>
              <h2 class='subtitle is-5 has-text-danger'>
              <form action='record/delete/${id}' method='POST'>            
                  <input type='text' hidden name='declaration'
                  <input class='input' type='text'
                  name='typedDeclaration' placeholder
                                      ='Type Declaration'>
                  <p class="help is-danger">${getError(errors,
                  <button class='button is-primary'>Delete</button>


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


const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateDeclaration : check('typedDeclaration')
    // To delete leading and trailing space
    // Custom validator
    // Check string matches with comparison
    // (some exact value) or not
    .custom(async (typedDeclaration, {req}) => {
      const declaration = req.body.declaration
      if( typedDeclaration !== declaration){
        throw new Error('Please type exact declaration')


Filename – package.json

package.json file




Attempt to submit the form when input is not exactly equals to the declaration

Response when attempt to submit the form when input is not exactly equals to the declaration

Attempt to submit the form when input is  exactly equals to the declaration

Response when attempt to submit the form when input is exactly equals to the declaration

Database after successful submission of form:

Database after successful submission of form

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

Last Updated : 25 Jan, 2023
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