Code to check a CNPJ number and automatically suggest fixes in case of a single digit typo

readme.md

If the code looks like it was generated by ChatGPT, is becaust it was: I wanted a quick soluton as I got a mistyped CNPJ in a message and I had to find the correct one. ChatGPT provided one and I'm happy with the result.

I'm publishing this (after some cleaning of its code to add modularity and improve some issues) as I think it should be a standard feature on most (if not all) websites asking for a CNPJ number. In fact, I think the same code could be adapted for CPF too. The way it implemented variations is crude, but since there is just 140-ish checks to be made, the code is fast. If I were to implement it myself I would try to use algebra, but brute forcing worked here.

If you have Deno installed, you can try out the example by running this command:

deno run -A https://gist.github.com/qgustavor/193ebe5ddedffe45a9897955625be90e/raw/check-cnpj-example.js

Which will return

The CNPJ 01.005.727/0001-23 is invalid.
Did you mean:
01.005.727/0001-24

check-cnpj-example.js

import { checkCNPJ } from './check-cnpj.js'

const userCNPJ = '01.005.727/0001-23'
const result = checkCNPJ(userCNPJ)

if (result.valid) {
  console.log(`The CNPJ ${result.formattedCNPJ} is valid.`)
} else {
  console.log(`The CNPJ ${result.formattedCNPJ} is invalid.`)
  if (result.possibleCorrections.length > 0) {
    console.log('Did you mean:')
    result.possibleCorrections.forEach(cnpj => console.log(cnpj))
  }
}

check-cnpj.js

export function validateCNPJ (cnpj) {
  // Remove non-numeric characters
  cnpj = cnpj.replace(/\D/g, '')

  // Check if the CNPJ has 14 digits
  if (cnpj.length !== 14) {
    return false
  }

  // Define the weights for the calculations
  const firstDigitWeights = [5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2]
  const secondDigitWeights = [6, 5, 4, 3, 2, 9, 8, 7, 6, 5, 4, 3, 2]

  // Calculate the first check digit
  let sum = 0
  for (let i = 0; i < 12; i++) {
    sum += parseInt(cnpj[i]) * firstDigitWeights[i]
  }
  let remainder = sum % 11
  const firstCheckDigit = remainder < 2 ? 0 : 11 - remainder

  // Calculate the second check digit
  sum = 0
  for (let i = 0; i < 13; i++) {
    sum += parseInt(cnpj[i]) * secondDigitWeights[i]
  }
  remainder = sum % 11
  const secondCheckDigit = remainder < 2 ? 0 : 11 - remainder

  // Check if the check digits are correct
  return Number(cnpj[12]) === firstCheckDigit && Number(cnpj[13]) === secondCheckDigit
}

export function generateVariations (cnpj) {
  cnpj = cnpj.replace(/\D/g, '')
  const validCNPJs = []
  if (cnpj.length !== 14) return validCNPJs
  
  // That's crude, but works
  for (let i = 0; i < 14; i++) {
    for (let digit = 0; digit <= 9; digit++) {
      if (Number(cnpj[i]) !== digit) {
        const cnpjVariation = cnpj.substring(0, i) + digit + cnpj.substring(i + 1)
        if (validateCNPJ(cnpjVariation)) {
          validCNPJs.push(cnpjVariation)
        }
      }
    }
  }

  return validCNPJs
}

export function formatCNPJ (cnpj) {
  return cnpj.replace(/\D/g, '').replace(/^(\d{2})(\d{3})(\d{3})(\d{4})(\d{2})$/, '$1.$2.$3/$4-$5')
}

export function checkCNPJ (cnpj) {
  const isValid = validateCNPJ(cnpj)

  if (isValid) {
    return {
      valid: true,
      formattedCNPJ: formatCNPJ(cnpj),
      possibleCorrections: []
    }
  }

  const variations = generateVariations(cnpj)
  const formattedVariations = variations.map(formatCNPJ)

  return {
    valid: false,
    formattedCNPJ: formatCNPJ(cnpj),
    possibleCorrections: formattedVariations
  }
}