promise封装

<script>
    var promise = new Promise((resolve, reject) => {
       //resolve成功的处理方式，reject失败的处理方式

    })

    function getJson(url) {
       const promise = new Promise((reslove, reject) => {
          //网络请求
          const client = new XMLHttpRequest()
          client.open("get", url)
          client.responseType = "json"
          client.send()
          client.onreadystatechange = function() {
             if (client.readyState === 4) {
                if (client.status === 200) {
                   reslove(client.response)
                } else {
                   reject(new Error(client.statusText))
                }
             }
          }
       })
       return promise
    }
</script>