can't get xmlhttprequest working on nodemcu

Hi, forgive me for my bad english.

I'm using a nodemcu devkit 0.9 with espruino v1.89. When I add these lines, my board seems to crash. I can't get the prompt and i have to disconnect en reconnect the board to usb, then make a reset. If i remove these lines, everything is fine.

'<script>'+
            'function postForm(e) {'+
              'e.preventDefault();'+
              'var mode = document.getElementById("mode").value;'+
              'var hostname = document.getElementById("hostname").value;'+
              'var ssid = document.getElementById("wifiSSID").value;'+
              'var pass = document.getElementById("wifiPass").value;'+
              'var data = { "mode": mode, "hostname": hostname, "ssid": ssid, "pass": pass };'+
              'var xmlhttp = new XMLHttpRequest();'+
              'xmlhttp.open("POST", "/save");'+
              'xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8");'+
              'xmlhttp.send(JSON.stringify(data);'+
            '}'+

This is the memory without these lines:
={ "free": 1332, "usage": 368, "total": 1700, "history": 383 }

So it's not a problem with memory limit. It seems to be a problem with xmlhttp.open, xmlhttp.setRequestHeader and xmlhttp.send. The problem appears just when I add the 3 lines. Any idea ??