Edit in JSFiddle

function go() {
  var lat = parseFloat(document.getElementById("lat").value);
  var lon = parseFloat(document.getElementById("lon").value);
  var typeIdx = document.getElementById("type").selectedIndex;
  var fo = {
    type: [document.getElementById("type").options[typeIdx].value]
  } ;
  var rayon= parseFloat(document.getElementById("rayon").value);
  if (rayon) {
    fo.circle= {
        x: lon,
        y: lat,
        radius: rayon
    } ;
  }
  var resultDiv = document.getElementById("result");
  try {
    Gp.Services.reverseGeocode({
      position: {
        x: lon,
        y: lat
      },
			srs: 'EPSG:4326',
      filterOptions: fo,
      onSuccess: function(result) {
        resultDiv.innerHTML = "<p>" + JSON.stringify(result) + "</p>"
      },
      onFailure: function(error) {
        resultDiv.innerHTML = "<p>" + error + "</p>"
      }
    });
  } catch (e) {
    resultDiv.innerHTML = "<p>" + e + "</p>"
  }
}
var infoDiv = document.getElementById("info");
infoDiv.innerHTML = "<p> Bibliothèque d'accès version " + Gp.servicesVersion + " (" + Gp.servicesDate + ")</p>";

<div id="params">
  <p>
    longitude :
    <input type="text" id="lon" size="10" /> latitude :
    <input type="text" id="lat" size="10" />
  </p>
  <p>Type de Géocodage :
    <select id="type">
      <option value="PositionOfInterest">Toponymes</option>
      <option value="StreetAddress">Adresses</option>
      <option value="CadastralParcel">Parcelles Cadastrales</option>
    </select>
  </p>
  <p>
    rayon de recherche :
    <input type="text" id="rayon" size="5" />
  </p>
</div>
<div id="go">
  <input type="button" value="Géocodage Inverse" onclick="go()" />
</div>
<div id="result"></div>
<div id="info"></div>

#params {
  width: 100%;
  height: 140px;
  box-shadow: 0 0 10px #999;
  font-family: monospace;
  padding: 5px;
}

#go {
  padding: 5px;
  float: center;
  width: 100%;
  height: 30px;
}

#result {
  padding: 5px;
  width: 100%;
  height: 200px;
  box-shadow: 0 0 10px #999;
  font-family: monospace;
  overflow: scroll;
}
#info {
  padding: 5px;
  width: 100%;
  height: 20px;
  font-family: monospace;
}