xr是XMLHttpRequest的超简单封装，并返回ES6Promise。
示例代码：
xr.get('/api/items', {take: 5}) .the(res => cosole.log(res.data));xr.post('/api/item', {ame: 'hello'}) .the(res => cosole.log("ew item", res.data));xr({ method: xr.Methods.GET, url: '/api/items', params: {take: 5}, evets: { progress: (xhr, xhrProgressEvet) => { cosole.log("xhr", xhr); cosole.log("progress", xhrProgressEvet); } }});
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