promise对象封装ajax

function getJson(url) {
    return new Promise((resolve, reject) => {
        let xhr = new XMLHttpRequest();
        xhr.open('GET', url, true);

        xhr.onreadystatechange = function () {
            if (this.readyState === 4) {
                if (this.status === 200) {
                    resolve(this.responseText, this)
                } else {
                    let resJson = {
                        code: this.status,
                        response: this.response
                    }
                    reject(resJson, this)
                }
            }

        }
        xhr.send();
    })
}

  function postJson(url, data) {
    return new Promise((resole, reject) => {
        let xhr = new XMLHttpRequest();
        xhr.open('POST', url, true);
        xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');

        xhr.onreadystatechange = function () {
            if (this.readyState === 4) {
                if (this.status === 200) {
                    resolve(JSON.parse(this.responseText), this)
                } else {
                    let resJson = {
                        code: this.status,
                        response: this.response
                    }
                }
            }
        }
        xhr.send(JSON.stringify(data))
    })
}

getJSON('/api/v1/xxx')    // => 这里面是就try
.catch( error => {
    // dosomething          // => 这里就是catch到了error，如果处理error以及返还合适的值
})
.then( value => {
    // dosomething          // 这里就是final
})