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This documentation relates to Opera's now deprecated .oex Extension API framework for Opera versions <= 12.15 and also provided by our OEX2NEX shim library.

For the latest Opera Extensions API documentation for Opera versions > 12.15 please consult the latest Opera Extensions API documentation online.


The readonly id attribute is a unique identifier for this browser window. The value is a string of characters determined by the implementation and is unique for each browsing session.


readonly DOMString id


The following example creates a button on the browser toolbar. When the button is clicked, the ID of the last focused window is displayed in the button's badge.

// The background process (e.g. index.html)

// Specify the properties of the button before creating it.
var UIItemProperties = {
  disabled: false,
  title: "Example extension",
  icon: "images/icon_18.png",
  badge: {
    backgroundColor: '#cc0000',
    color: '#ffffff'

// Create the button and add it to the toolbar.
var button = opera.contexts.toolbar.createItem( UIItemProperties );  

button.addEventListener('click', handleClick, false);

function handleClick() {
  // Get the last focused window
  var thisWin = opera.extension.windows.getLastFocused();
  // Display the window's ID in the button's badge
  button.badge.textContent = thisWin.id;