The Window Object - JavaScript: Novice to Ninja - page 292

In general, you should refer to global variables without using the window object (it's less
typing and your code is more portable between environments). An exception is if you need
to check whether a global variable has been defined. For example, the following code will
throw an exception if x has not been defined:
if (x) {
// do something
}
However, if the variable is accessed as a property of the window object, an exception will
not occur (although the block of code will still not be evaluated without x being defined):
if (window.x) {
// do something
}
Some functions that we've already met such as parseInt() and isNaN() are global
functions, which in a browser environment makes them methods of the window object:
window.parseInt(4.2);
<< 4
window.isNaN(4.2);
<< false
Like variables, it is customary to omit accessing them through the window object.
Dialogs
In Chapter 1, we introduced three functions that produced dialogs in the browsers:
alert() , confirm() , and prompt() . These are not part of the ECMAScript stand-
ard, although all major browsers support them as methods of the window object.
The window.alert() method will stop the execution of the program and display a mes-
sage in a dialog box. The message is provided as an argument to the method and un-
defined is always returned: