前端异步方案-Promise

function ajax(url) {
    return new Promise(function (resolve,reject) {
        var xml = new XMLHttpRequest()
        xml.open('GET', url, true);
        xml.send(null);
        xml.onreadystatechange = function () {
            if (xml.readyState === 4) {
                if (xml.status === 200) {
                    // 请求成功
                    resolve(xml.responseText)                        
                } else {
                    reject(new Error('请求失败状态码:'+ xml.status))
                }
            }
        }
    })
}

ajax('http://www.xxxx.com').then(res-0 =>{
    console.log(res-0)
    return ajax('http://www.ccc.com')
}).then(res-1 =>{
    console.log(res-1)
}).catch(err =>{
    console.log(err)
})

module.exports = {
    defered() {
        var _success, _error;
        return {
            resolve(res) {
                _success(res)
            },
            reject(err) {
                _error(err) 
            },
            promise: {
                then(success, error){
                    _success = success
                    _error = error
                }
            }
        }
    }
}