ajax和promise实例

const getJSON = function(url) {
const promise = new Promise(function(resolve, reject){
const handler = function() {
if (this.readyState !== 4) {
return;
}
if (this.status === 200) {
resolve(this.response);
} else {
reject(new Error(this.statusText));
}
};
const client = new XMLHttpRequest();
client.open("GET", url);
client.onreadystatechange = handler;
client.responseType = "json";
client.setRequestHeader("Accept", "application/json");
client.send();

});

return promise;
};

getJSON("/posts.json").then(function(json) {
console.log('Contents: ' + json);
}, function(error) {
console.error('出错了', error);
});