In depth analysis of the role of top in js

Every WINDOW object, whether it's a regular HTML page. Frameset page. Both the subframe and the grandchild frame have a TOP attribute. This property returns a reference to the top-level WINDOE object loaded in the browser;

. If the WINDOW object is a regular HTML page, TOP is SELF
. Top level frame and page...
. Subframe, TOP is the parent of the frame, that is, the following two expressions are equivalent;
The SELF. The TOP
The SELF. The PARENT