Is window.document ever null or undefined?

dom, javascript

Solution

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:

- the jQuery ready function fails (likely in at least some browsers, though not the ones in popular use for desktop and some mobile devices),

- there is no `window object`, or

- 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));

Problem

Original source

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