异步上传文件，在很多场景中都有应用。本文介绍一种通过xhr和formdata实现的方法。

FormData属于h5的内容所以，浏览器兼容不是很好，下面是一种polyfill，inspired by francois2metz/html5-formdata

(function(w) {
    if (w.FormData)
        return;
    function FormData() {
        this.boundary = "--------FormData" + Math.random();
        this._fields = [];
    }
    FormData.prototype.append = function(key, value) {
        this._fields.push([key, value]);
    }
    FormData.prototype.toString = function() {
        var boundary = this.boundary;
        var body = "";
        this._fields.forEach(function(field) {
            body += "--" + boundary + "\r\n";
            // file upload
            if (field[1].name) {
                var file = field[1];
                body += "Content-Disposition: form-data; name=\""+ field[0] +"\"; filename=\""+ file.name +"\"\r\n";
                body += "Content-Type: "+ file.type +"\r\n\r\n";
                body += file.getAsBinary() + "\r\n";
            } else {
                body += "Content-Disposition: form-data; name=\""+ field[0] +"\";\r\n\r\n";
                body += field[1] + "\r\n";
            }
        });
        body += "--" + boundary +"--";
        return body;
    }
    w.FormData = FormData;
    var _send = XMLHttpRequest.prototype.send;
    XMLHttpRequest.prototype.send = function(data) {
        if (data instanceof FormData) {
            xhr.setRequestHeader("Content-Type", "multipart/form-data;boundary="+ data.boundary);
            return xhr.sendAsBinary(data.toString());
        }
        return _send.call(this, data);
    };
})(window);

像这样使用

var data = new FormData();
data.append('file', document.getElementById('file').files[0]);
xhr.open('POST', '/', true);
xhr.onprogress = (function(){
    var progress = 0;
    return function(evt){
        var p = ~~(evt.loaded*1000/evt.total)
        p /= 1000
        if(progress !=== p){
            progress = p
            console.log('progress: ', p)
        }
    }
})();

关于兼容性，只要支持FieApi就没问题