Is window.document ever null or undefined?

Is there ever a case when the window.document object is null or undefined?

Yes, for JavaScript code that isn’t in a document (e.g. node.js). But such code may not have a window object either (though it will have a global object).

For code in HTML documents in user agents compliant with the W3C DOM, no.

> Are there any situations in which this piece of code will fail (i.e. throw
> an exception)?
> 
> [snip jQuery code]

It will fail where:

1. the jQuery ready function fails (likely in at least some browsers, though not the ones in popular use for desktop and some mobile devices),
2. there is no window object, or
3. there is no window.document object

To be confident of code working in a variety of hosts, you can do something like:

  if (typeof window != 'undefined' && window.document &&
      window.document.readyState == whatever) {
      // do stuff
  }

which isn’t much extra to write, and likely only needs to be done once.

Alternatives:

(function (global) {
  var window = global;
  if (window.document && window.document.readyState == whatever) {
    // do stuff
  }
}(this));

and

(function (global) {
  var window = global;

  function checkState() {
    if (window.document && window.document.readyState) {
      alert(window.document.readyState);
    } else {
      // analyse environment 
    }
  }
  // trivial use for demonstration
  checkState();
  setTimeout(checkState, 1000);
}(this));