15. Ajax

Ajax 简易实现：

export function getJSON(url) {
  return new Promise((resolve, reject) => {
    const xhr = XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP')
    xhr.open('GET', url, false)
    xhr.setRequestHeader('Accept', 'application/json')
    xhr.onreadystatechange = function () {
      if (xhr.readyState !== 4)
        return
      if (xhr.status === 200 || xhr.status === 304)
        resolve(xhr.responseText)
      else
        reject(new Error(xhr.responseText))
    }
    xhr.send()
  })
}