<!DOCTYPE html>
<html>
  <head>
    <title>JSON client</title>
    <style>
      :root {
        --bCol:#333;--cCol:#222;--dCol:#666;--tCol:#fff;
      }
      body {
        font-family: Verdana, sans-serif;
        text-align: center;
        background: var(--cCol);
        color: var(--tCol);
        margin: 20px;
        background-attachment: fixed;
      }
      button {
        background: var(--cCol);
        color: var(--tCol);
        border: 0.3ch solid var(--cCol);
        display: inline-block;
        font-size: 20px;
        margin: 8px;
        margin-top: 12px;
      }
      input {
        background: var(--cCol);
        color: var(--tCol);
        border: 0.5ch solid var(--cCol);
        width: 100%;
      }
      h1{
        margin: 0px;
        font-size: 20px;
      }
      h2{
        font-size: 16px;
        margin-top: 20px;
      }
      form{
        background: var(--bCol);
        width: 500px;
        padding: 20px;
        -webkit-border-radius: 10px;
        -moz-border-radius: 10px;
        display: inline-block;
      }
      textarea{
        background: var(--cCol);
        color: var(--tCol);
        padding-top: 10px;
        width: 100%;
        font-family: monaco,monospace;
        font-size: 12px;
        -webkit-border-radius: 10px;
        -moz-border-radius: 10px;
      }
    </style>
  </head>

  <body>
      <form name="cf">
          <h1>JSON API test tool</h1>
          <h2>URL:</h2>
          <input name="cu" type="text" size="60" value="http://192.168.4.1/json">
          <div id="buttons">
              <button type="button" onclick="rq('GET')">GET</button>
              <button type="button" onclick="rq('POST')">POST</button>
          </div>
          <h2>Body:</h2>
          <textarea name="bd" rows="8" cols="100"></textarea>
          <h2>Response:</h2>
          <textarea name="rsp" rows="25" cols="100"></textarea>
      </form>
  </body>
</html>

<script>
function rq(cm)
{
  var h = new XMLHttpRequest();
  h.open(cm, document.cf.cu.value, true);
  h.onreadystatechange = function()
  {
    if(h.readyState == 4)
    {
      if(h.status==200)
      {
        document.cf.rsp.value="Bad JSON: "+h.responseText
        document.cf.rsp.value=JSON.stringify(JSON.parse(h.responseText), null, '\t');
      }
      else
      {
        document.cf.rsp.value="Error "+h.status+"\r\n\n"+h.responseText;
      }
    }
  }
  h.send(document.cf.bd.value);
}
</script>