Extracting the hostname from a host string in a NodeJS request header.

hostnameof.js

function hostnameof(req) {
  var host = req.headers.host

  if (!host) {
    return
  }

  var offset = host[0] === '['
    ? host.indexOf(']') + 1
    : 0
  var index = host.indexOf(':', offset)

  return index !== -1
    ? host.substring(0, index)
    : host
}