I have an ASUS laptop.
Does this iMac support to be an external monitor to display the content in my laptop?
No, only some 27" models support DisplayPort Input.
see > Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display
How about 2012 the new version iMac 21.5inch?
Use Screen Sharing instead, it offers you many more possibilities including being able to use both machines, still.
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