test.ashx

<%@ WebHandler Language="C#" Class="API20261030090400" %>

// 注意，檔案編碼必須為 UTF-8-BOM 

using System;
using System.Collections.Generic; // Dictionary
using System.IO;
using System.Web;
using System.Web.Script.Serialization; // JavaScriptSerializer

public class API20261030090400 : IHttpHandler
{
  public bool IsReusable { get { return false; } }
  
  public void ProcessRequest(HttpContext context)
  {
    context.Response.ContentType="application/json; charset=utf-8";
    
    JavaScriptSerializer oJSS=new JavaScriptSerializer();
    
    Dictionary<string, object> oOutput=new Dictionary<string, object>();
    oOutput["booSuccess"]=true;
    oOutput["strErrorMessage"]="";
    
    try
    {
      string strInput="";
      using (StreamReader oSR=new StreamReader(context.Request.InputStream)) { strInput=oSR.ReadToEnd(); }
      Dictionary<string, object> oInput=oJSS.Deserialize<Dictionary<string, object>>(strInput);
      oOutput["oInput"]=oInput;
      
      string[] straFullPath=Directory.GetFiles(context.Server.MapPath("~/text"));
      Array.Sort(straFullPath);
      Array.Reverse(straFullPath);
      
      List<string> lstFile=new List<string>();
      List<string> lstContent=new List<string>();
      for(int i=0; i<straFullPath.Length; i++)
      {
        string strFullPath=straFullPath[i];
        
        lstFile.Add(Path.GetFileName(strFullPath));
        lstContent.Add(File.ReadAllText(strFullPath));
      }
      
      string[] straFile=lstFile.ToArray();
      string[] straContent=lstContent.ToArray();
      
      oOutput["straFile"]=straFile;
      oOutput["straContent"]=straContent;
    }
    catch (Exception oE)
    {
      context.Response.StatusCode=500;
      oOutput["booSuccess"]=false;
      oOutput["strErrorMessage"]=HttpUtility.JavaScriptStringEncode(oE.Message);
    }
    finally
    {
      string strOutput=oJSS.Serialize(oOutput);
      context.Response.Write(strOutput);
    }
  }
}

// 若要偵錯時，記得在網站根目錄的 web.config 加入 system.web 區塊
// 
// <?xml version="1.0" encoding="UTF-8"?>
// <configuration>
// 
//   <system.web>
//     <customErrors mode="Off" />
//     <compilation debug="true" />
//   </system.web>
//   
//   <system.webServer>
//     <httpErrors>
//       <remove statusCode="404" subStatusCode="-1" />
//       <error statusCode="404" prefixLanguageFilePath="" path="/404.htm" responseMode="ExecuteURL" />
//     </httpErrors>
//   </system.webServer>
//   
// </configuration>
// 

test.htm

<!doctype html>
<html lang="zh-Hant-TW">
  <head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8">
    <title>your title</title>
  </head>
  <body>
    <script>
window.addEventListener(
  "load",
  async function()
  {

    while(true)
    {
      try
      {
        var oResponse=await fetch(
          "test.ashx",
          {
            method: "POST",
            headers: { "Content-Type": "application/json; charset=UTF-8" },
            body: JSON.stringify({"strUsername":"john", "strPassword":"123"})
          }
        );
        
        var strJson=await oResponse.text();
        var oJson=JSON.parse(strJson);
        
        console.log(oJson);
      }
      catch(oError)
      {
        console.log({ "oResponse":oResponse, "strJson":strJson, "oError":oError });
        alert((oResponse.url+"\n"+oResponse.status+"\n"+oResponse.statusText)+"\n\n"+strJson+"\r\n"+(oError.toString()));
        break;
      }
      
      break;
    }

  }
);
    </script>
  </body>
</html>