First, the concept of BOM
BOM (Browser Object Model) object model refers to the browser, the browser object model provides independently of the content of the object structure can interact with the browser window.
BOM by multiple objects, which represents the browser window Window object is a top-level BOM objects, other objects are sub-objects of the object.
Two, BOM's top-level object
window is the top of the Object Browser, when calling the properties and methods under the window, you can be omitted window.
Global variables defined in the <script> tag in the window belong, are attributes of the object window.
note:
1. name is a window attribute is a string type, do not use all of the attributes or variables in order to name
2. top is a window of mathematics, can only get the value can not be assigned.