is-valid-domain/index.js

const sldMap = require('./domains/sld')

module.exports = function isValidDomain(value, opts) {
  if (typeof value !== 'string') return false
  if (!(opts instanceof Object)) opts = {}
  value = value.toLowerCase()

  if (value.endsWith('.')) {
    value = value.slice(0, value.length-1)
  }

  if (value.length > 253) {
    return false
  }

  const validChars = /^([a-z0-9-._*]+)$/g
  if (!validChars.test(value)) {
    return false
  }

  const sldRegex = /(.*)\.(([a-z0-9]+)(\.[a-z0-9]+))/
  const matches = value.match(sldRegex)
  var tld = null
  var labels = null
  if (matches && matches.length > 2) {
    if (sldMap[matches[2]]) {
      tld = matches[2]
      labels = matches[1].split('.')
    }
  }

  if (!labels) {
    labels = value.split('.')
    if (labels.length <= 1) return false

    tld = labels.pop()
    const tldRegex = /^(?:xn--)?(?!^\d+$)[a-z0-9]+$/gi

    if (!tldRegex.test(tld)) return false
  }

  if (opts.subdomain == false && labels.length > 1) return false

  const isValid = labels.every(function(label, index) {
    if (opts.wildcard && index === 0 && label === '*' && labels.length > 1) {
      return true
    }

    let validLabelChars = /^([a-zA-Z0-9-_]+)$/g
    if (index === labels.length - 1) {
      validLabelChars = /^([a-zA-Z0-9-]+)$/g
    }

    const doubleDashCount = (label.match(/--/g) || []).length
    const xnDashCount = (label.match(/xn--/g) || []).length
    if (doubleDashCount !== xnDashCount) {
      return false
    }

    const isValid = (
      validLabelChars.test(label) &&
      label.length < 64 &&
      !label.startsWith('-') &&
      !label.endsWith('-')
    )

    return isValid
  })

  return isValid
}