Editing WoW:API GetScreenWidth (section)

{{wowapi}}
Returns the width of the window in pixels.

 GetScreenWidth();

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;''Arguments''

:''none''

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;''Returns''

:Number screenWidth

:;screenWidth : Width of window in pixels

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;''Example''
 if( GetScreenWidth() * UIParent:GetEffectiveScale() == 800) then ChatFrame:AddMessage("Using 800x600 mode"); end

;''Result''
 Using 800x600 mode

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;''Description''

: Returns the width of the window in pixels.  This value is affected by the UI's scale.

: Also note that the return value will not match the screen resolution chosen from the Video Options menu, but will instead represent a dimension with the same aspect ratio.  For example, the following code...
 DEFAULT_CHAT_FRAME:AddMessage( ( GetScreenWidth() * UIParent:GetEffectiveScale() ).."x"..( GetScreenHeight() * UIParent:GetEffectiveScale() ) );
: ...will print "960x768" for the actual screen resolution "1280x1024".  Both of these dimensions share a 5:4 aspect ratio.