In "strict mode", "this" is not set anymore to the global context in a function. It remains "undefined" for security reasons.

In other words, the following code should result in "glob" set to "undefined" in browsers that support strict mode:

"use strict";
(function () {
  var glob = this; // "glob" is "undefined", not the global context
})();

This bug went unnoticed up until now because Web browsers simply did not implement that feature. Latest versions of Chrome, Firefox and possibly others apply that rule correctly, though, making the JavaScript crash.