How to validate if input in input field is divisible by or multiple of a number using express-validator ?

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 numbers that are multiples of a particular number are allowed i.e. there not allowed any letters, special characters, or a number that is not multiple of a particular number. We can also validate these input fields to accept only those particular numbers 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 isDivisibleBy(number) with ‘ . ‘
  • Sanitize the input to convert it from string to integer by chaining on the sanitization method toInt() 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 numbers those are divisible by some particular number.



Filename – index.js

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const express = require('express')
const bodyParser = require('body-parser')
const {validationResult} = require('express-validator')
const repo = require('./repository')
const { validateAmount } = 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) => {
  res.send(formTemplet({}))
})
  
// Post route to handle form submission logic and 
app.post(
  '/info',
  [validateAmount],
  async (req, res) => {
    const errors = validationResult(req)
    if(!errors.isEmpty()){
      return res.send(formTemplet({errors}))
    }
    const {name, ano, pin, amount} = req.body
  
    // New record
    await repo.create({
      'Account Name':name,
      'Account Number':ano,
      'Secret Password':pin,
      'Amount':amount,
    })
    res.send(
'<strong>Transaction successfull!</strong>')
})
  
// Server set to run
app.listen(port, () => {
  console.log(`Server start on port ${port}`)
})

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Filename – repository.js: This file contains all the logic to create a local database and interact with it.

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// 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 {
      fs.accessSync(this.filename)
    } 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
    records.push(attrs)
    await fs.promises.writeFile(
      this.filename,
      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 formet.
module.exports = new Repository('datastore.json')

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Filename – form.js: This file contains logic to show the form to submit the data.

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const getError = (errors, prop) => {
  try {
    return errors.mapped()[prop].msg
  } catch (error) {
    return ''
  
}
  
module.exports = ({errors}) => {
  return `
<!DOCTYPE html>
<html>
  
<head>
  <link rel='stylesheet' href=
  <style>
    div.columns {
      margin-top: 100px;
    }
  
    .button {
      margin-top: 10px
    }
  </style>
</head>
  
<body>
  <div class='container'>
    <div class='columns is-centered'>
      <div class='column is-5'>
        <form action='/info' method='POST'>
          <div>
            <div>
              <label class='label' id='name'>
                Name
              </label>
            </div>
            <input class='input' type='text' 
              name='name' placeholder='Vinit singh' 
              for='name'>
          </div>
          <div>
            <div>
              <label class='label' id='ano'>
                Account number
              </label>
            </div>
            <input class='input' type='text' 
              name='ano' placeholder='Account Nnumber'
              for='ano'>
          </div>
          <div>
            <div>
              <label class='label' id='pin'>
                Secret Password
              </label>
            </div>
            <input class='input' type='text' name='pin'
              placeholder='XXXXXX' for='pin'>
          </div>
          <div>
            <div>
              <label class='label' id='amount'>
                Amount
              </label>
            </div>
            <input class='input' type='text' name='amount'
              placeholder='Amount in multiple of 100' 
              for='amount'>
            <p class="help is-danger">
              ${getError(errors, 'amount')}
            </p>
          </div>
          <div>
            <button class='button is-primary'>
              Submit
            </button>
          </div>
        </form>
      </div>
    </div>
  </div>
</body>
  
</html>
  `
}

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

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const {check} = require('express-validator')
const repo = require('./repository')
module.exports = {
    
  validateAmount : check('amount')
  
    // To delete leading and trailing space
    .trim()
  
    // Convert amount to integer from string
    .toInt()
  
    // Validate name to accept only a 
    // number divisible by 'num'
    .isDivisibleBy(100)
  
    // Custom message
    .withMessage('Must be multiple of 100')   
}

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Filename – package.json

package.json file

Database:

database

Output:

Attempt to submit form data when amount input field is not divisible by or multiple of 100


Response when attempt to submit form data where amount input field is not divisible by or multiple of 100


Attempt to submit form data when amount input field is divisible by or multiple of 100


Response when attempt to submit form data where amount input field is divisible by or multiple of 100

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.




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