Example showing use of Logger passthrough in client-side JavaScript. See https://mogsdad.wordpress.com/?p=193.

Logger-Sidebar.html

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script>
  /**
   * On document load, assign click handlers to button(s), add
   * elements that should start hidden (avoids "flashing"), and
   * start polling for document selections.
   */
  $(function() {
    // assign click handler(s)
    $('#submit-button').click(function() {
      Logger.log("submit-button clicked");
      $('#submit-button').attr('disabled',true);
      Logger.log(JSON.stringify($('#input-form').serializeArray()));
      // ...and then do something useful
    });

    // Get static server-side info
    google.script.run
     .withSuccessHandler(
       function(value) {
        Logger.log("Received initial info "+JSON.stringify(value));
        $('#submit-button').attr('disabled',false);
       })
     .withFailureHandler(buttonBarErr)
     .getInitialInfo();
    
    // Add elements that should start hidden

  });


  /**
   * Display an error message below buttonBar.
   */
  function buttonBarErr(msg, element) {
    var caughtErrors = [
      "ScriptError: We're sorry, a server error occurred. Please wait a bit and try again.",
      "NetworkError: Connection failure due to HTTP",
      "ScriptError: Service error"
    ];
    var reportError = true;

    // Don't report any of our caughtErrors
    for (var i=caughtErrors.length-1; i>=0 && reportError; i--) {
      var t = String(msg).indexOf(caughtErrors[i]);
      reportError = (String(msg).indexOf(caughtErrors[i]) === -1);
    }
    if (reportError) {
      showError(msg, $('#button-bar'));
      element.disabled = false;
    }
  }

</script>