封装简易AJAX

function ajax (url) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest()
    xhr.open('GET', url, true)
    xhr.onreadystatechange = function () {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          resolve(xhr.responseText)
        } else if (xhr.status === 404) {
          reject(new Error('error: status 404'))
        }
      }
    }
    xhr.send(null)
  })
}