This is a fairly common pattern for setting options, even if options isn't provided:

function doSomethingCool(options) {
  options = $.extend(true, {}, defaultOptions, options);
  // ...
}

...but it relies on the behavior that $.extend will ignore the options argument if it's null or undefined. $.extend does indeed do that, and on purpose (with a comment, even), but it's not documented.

Recommend documenting it by adding this text:

In the arguments list, null and undefined ''objectN'' arguments are skipped (they do not cause an error). For example, this works even if the options variable is undefined or null: \\\\ options = $.extend(true, {}, defaultOptions, options);

...just before the paragraph starting with "On a deep extend..." near the end.