index.js

 'use strict';

 const got = require('got');

 const statusCodes = {

  isUp: 1,

  isDown: 2,

  invalidDomain: 3

 };

 module.exports = async url => {

  const {body} = await got(`https://isitup.org/${url}.json`, {

  json: true,

  headers: {

  'user-agent': 'https://github.com/sindresorhus/is-up'

  }

  });

  const statusCode = body.status_code;

  if (statusCode === statusCodes.invalidDomain) {

  throw new Error('Invalid domain');

  }

  return statusCode === statusCodes.isUp;

 };