Validation for CPF and CNPJ

validateCpfCnpj.js

const isString = value => typeof value === 'string'
const format = value => value.replace(/[^\d]+/g, '')
const isValidNumber = (value, count) => format(value).length === count && !format(value).match(/(\d)\1{10}/)

const validator = value => format(value)
    .split('')
    .splice(format(value).length - 2)
    .map( el => +el )
const toValidate = (value, end, start = 0) => format(value)
    .split('')
    .filter((digit, index, array) => index >= start && index <= end && digit)
    .map(el => +el)

const sum = (array, start) => array.reduce((total, el, i) => total + el * (start - i), 0)
const rest = value => value % 11

const validate = (firstDigit, lastDigit, validator) => firstDigit === validator[0] && lastDigit === validator[1]

function isValidCPF(cpf) {
    if (!isString(cpf) || !isValidNumber(cpf, 11)) return false
    
    digit = (end, factor) => rest(sum(toValidate(cpf,end),factor) * 10) % 10

    const firstDigit = digit(8, 10)
    const lastDigit = digit(9, 11)

    return validate(firstDigit, lastDigit, validator(cpf))
}

function isValidCNPJ(cnpj) {
    if (!isString(cnpj) || !isValidNumber(cnpj, 14)) return false
    
    const digit = sum => rest(sum) < 2 ? 0 : 11 - rest(sum)

    const firstDigit = digit(sum(toValidate(cnpj,3),5) + sum(toValidate(cnpj,11,4),9))
    const lastDigit = digit(sum(toValidate(cnpj,4),6) + sum(toValidate(cnpj,12,5),9))

    return validate(firstDigit, lastDigit, validator(cnpj))
}

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