Promise

function getURL(URL) {
return new Promise(function (resolve, reject) {
var req = new XMLHttpRequest();
req.open('GET', URL, true);
req.onload = function () {
if (req.status === 200) {
resolve(req.responseText);
} else {
reject(new Error(req.statusText));
}
};
req.onerror = function () {
reject(new Error(req.statusText));
};
req.send();
});
}
// 运行示例
var URL = "http://httpbin.org/get";
getURL(URL).then(function onFulfilled(value){
console.log(value);
}).catch(function onRejected(error){
console.error(error);
});