window.onload = function() {

​

  document.getElementById('btnSendMsg').onclick = sendMsg;

  sendMsg.blocked = 0;

  function sendMsg() {

      if (sendMsg.blocked === 0) {

          ++sendMsg.blocked;

          setTimeout(removeSendMsgBlock, 0); // If you want one second

          // ...other processing

          display("Sending message at " + new Date());

      }

  }

  function removeSendMsgBlock() {

      --sendMsg.blocked;

  }

  function display(msg) {

    var p = document.createElement('p');

    p.innerHTML = msg;

    document.body.appendChild(p);

  }

};