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How to validate if input in input field has base64 encoded string 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. If In a certain input field only base 64 encoded string are allowed i.e there not allowed any other form of string which not constitute base64 encoded string. We can also validate these input fields to accept only base 64 encoded string 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 isBase64() 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 accept only base 64 encoded string.

Filename – index.js


const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateBase64Data } = 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)
      return res.send(formTemplet({errors}))
    const {name, base64data} = req.body
    await repo.create({
 res.send('<h2>Base 64 data decoded and '
  + 'Stored successfully in the database</h2>')
// 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')
const { base64decode } = require('nodejs-base64');
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){
    // Fetch all existing records
    const records = await this.getAll()
    // Decode base64 encoded data
    const data = base64decode(attrs.base64data)
    // New record
    const record = {,
    // All the existing records with
    // new record push back to database
    await fs.promises.writeFile(
      JSON.stringify(records, null, 2)  
    return record
// 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 HTML form.


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='/data' method='POST'>           
                    <label class='label' id='name'>
                  <input class='input' type='text'
                         placeholder='Submitted By'
                    <label class='label' id='base64data'>
                      Base 64 data
                  <input class='input' type='text'
                         placeholder='Base 64 Encode data'
                  <p class="help is-danger">
                    ${getError(errors, 'base64data')}
                  <button class='button is-primary'>

Filename – validator.js: This file contain all the validation logic(Logic to validate a input field to accept only base encoded 64 string)


const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
  validateBase64Data : check('base64data')
    // To delete leading and trailing space
    // Validate input field to accept only base32 string
    // Custom message
    .withMessage('Must be a Base 64 encoded string')

Filename – package.json

package.json file



Online website that convert a normal string to base 64 string:

Online website that convert a normal string to base64 string(to show for which string we give base64 input)


Attempt to submit with invalid base64 string

Response when attempt to submit with invalid base 64 string

Attempt to submit with valid base64 string

Response when attempt to submit with valid base 64 string

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 : 27 Dec, 2021
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