html文件上传函数

//fileupload.js
function select_upload(zurl, zmulti, callback) {
    var zinput_file = document.createElement("input");
    zinput_file.type = "file";
    zinput_file.multiple = zmulti;
    zinput_file.onchange = function (e) {

        var form = new FormData();
        for (var zi = 0; zi < zinput_file.files.length; zi++) {
            form.append("file[" + zi.toString() + "]", zinput_file.files[zi]);
        }

        
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4 && xhr.status == 200) {
                callback(xhr.responseText);
            }
        };

        xhr.open("post", zurl);
        xhr.send(form);
        
    }
    zinput_file.click();
}

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title></title>
    <script src="fileupload.js"></script>
</head>
<body>
<input type="button" value="上传文件" id="btnupload" onclick="upload_file();" />
<script type="text/javascript">
    function upload_file() {
        
        select_upload("upload.aspx", true, function (text) {
            alert(text);
        });
    }
</script>
</body>
</html>

//upload.aspx
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace testfileupload
{
    public partial class upload : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Request.Files.Count > 0) {
                string zpath = Server.MapPath("upload");

                for (int i = 0; i < Request.Files.Count; i++) {
                    Request.Files[i].SaveAs(System.IO.Path.Combine(zpath, Request.Files[i].FileName));
                }
            }
            Response.Write(Request.Files.Count.ToString() + "文件上传了");
        }
    }
}

复杂版本的上传js函数

function select_upload(zurl, zmulti, sucess_callback ,ztag_name,open_callback,error_callback) {
    var zinput_file = document.createElement("input");
    zinput_file.type = "file";
    zinput_file.multiple = zmulti;
    if (!ztag_name) {
        ztag_name = "file";
    }
    zinput_file.onchange = function (e) {

        var form = new FormData();
        if (zmulti) {
            for (var zi = 0; zi < zinput_file.files.length; zi++) {
                form.append(ztag_name+"[" + zi.toString() + "]", zinput_file.files[zi]);
            }
        } else {
            form.append(ztag_name, zinput_file.files[0]);
        }

        if (open_callback) {
            open_callback("");
        }

        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                if ((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304) {
                    if (xhr.status == 200) {
                        sucess_callback(xhr.responseText);
                    }
                    
                } else {
                    if (error_callback) {
                        error_callback(xhr.status);
                    }
                }
            }       
        };

        xhr.open("post", zurl);
        xhr.send(form);

    }
    zinput_file.click();
}