The obsolete Document interface's fullscreen read-only property reports whether or not the document is currently displaying content in fullscreen mode.

Although this property is read-only, it will not throw if it is modified (even in strict mode); the setter is a no-operation and it will be ignored.

A Boolean value which is true if the document is currently displaying an element in fullscreen mode; otherwise, the value is false.

This simple function reports whether or not fullscreen mode is currently active, using the obsolete fullscreen property.

function isDocumentInFullScreenMode() {
  return document.fullscreen;
}

This next example, on the other hand, uses the current fullscreenElement property to determine the same thing:

function isDocumentInFullScreenMode() {
  return document.fullscreenElement !== null;
}

If fullscreenElement isn't null, this returns true, indicating that fullscreen mode is in effect.

• Fullscreen API
• Guide to the Fullscreen API
• Document.fullscreenEnabled