How To Get Coordinates of the google maps Corners?

I'm new in Google Maps! and I don't know how to ask this question

I need to draw polygon with just four coordinates that is for the four corners of the loaded map in zoom 13 on other hand get coordinates of the whole map to show to user. ( user search the specific area with draw a polygon on the map but if he/she don't draw a polygon i want to draw a polygon in size of the projected map for him/her and show the result. )

but i don't know how ?

sorry if you confiused !