Edit in JSFiddle

function go() {
  var lon1 = document.getElementById("lon1").value;
  var lat1 = document.getElementById("lat1").value;
  var lon2 = document.getElementById("lon2").value;
  var lat2 = document.getElementById("lat2").value;
  var sampling = document.getElementById("sampling").value;
  var resultDiv = document.getElementById("result");
  var positions = [];
  if (lon1 && lat1) positions.push({
    lon: lon1,
    lat: lat1
  });
  if (lon2 && lat2) positions.push({
    lon: lon2,
    lat: lat2
  });
  try {
    Gp.Services.getAltitude({
      positions: positions,
      sampling: sampling,
      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 1 :
    <input type="text" id="lon1" size="10" /> latitude 1 :
    <input type="text" id="lat1" size="10" />
  </p>
  <p>
    longitude 2 :
    <input type="text" id="lon2" size="10" /> latitude 2 :
    <input type="text" id="lat2" size="10" />
  </p>
  <p>
    sampling (nombre de points intermédaires) :
    <input type="text" id="sampling" size="5" />
  </p>
</div>
<div id="go">
  <input type="button" value="getAltitude" onclick="go()" />
</div>
<div id="result"></div>
<div id="info"></div>

#params {
  width: 100%;
  height: 150px;
  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;
}