Promise封装ajax

[Henry-Diasa]

function getJSON(url) {
    return new Promise((resolve, reject) => {
         let xhr = new XMLHttpRequest()
         xhr.open('GET', url, true)
         xhr.onreadystatechange = function() {
             if(this.readyState!== 4) return
             if(this.status === 200) {
                 resolve(this.response)
             } else {
                reject(new Error(this.statusText))
             }
         }
         xhr.onerror = function() {
           reject(new Error(this.statusText))
         }
         xhr.responseType = 'json'
         xhr.setRequestHeader('Accept', 'application/json')
         xhr.send(null)
    })
}