Home > Posts > Gotcha: var x = x() in Javascript gives “Object Expected” error

Gotcha: var x = x() in Javascript gives “Object Expected” error

At ClipFlair Studio (a Silverlight app), I had some time ago implemented a confirmation warning upon user trying to close the webpage (when it was running inside the web browser), which then had stopped functioning. It seems at some refactoring I had added code like the following:

var activityView = activityView();

and it was failing with error “Object Expected” (and even worse this was done silently – had to use browser’s debugging tools to catch it – because the respective code was running at onbeforeunload browser window event).

Such code would be fine in C#, but in Javascript it seems that a local variable with name x hides a function named x (with any number of parameters in its definition). This is obviously because functions are first-class objects in Javascript and can be treated like variables themselves too.

All started working again fine after changing that code to:

var a = activityView();

Below is the respective block of Javascript code included in a script tag at the webpage that hosts the Silverlight control. Note the “control.content.activityWindow”, where “activityWindow”  is accessed via the Silverlight HTMLBridge (that object – that has to be marked with ScriptableType class attribute – is registered using that key via HtmlPage.RegisterScriptableObject at the Silverlight app side).

function silverlightControl() {
  return document.getElementById("silverlightControl");
}

function onSilverlightLoad(sender, args) {
  var control = silverlightControl();
  if (control != null)
    control.focus();
}

function activityWindow() {
  var control = silverlightControl();
  if ( (control != null) && (control.content != null) )
    return control.content.activityWindow;
  else
    return null; //need this so that it doesn't return undefined
}

function activityView() {
  var a = activityWindow();
  if (a != null)
    return a.GetView();
  else
    return null; //need this so that it doesn't return undefined
}

function onClosing() {
  var a = activityView();
  if ( (a != null) && (a.WarnOnClosing) )
    return "Do you want to exit ClipFlair Studio?";
//else return undefined is implied (no onClosing message that is) } function onClosed() { var a = activityView(); if (a != null) a.WarnOnClosing = false; } function installEventHandlers() { window.onbeforeunload = onClosing; window.onunload = onClosed; } installEventHandlers();
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