Hi I just bought Macbook Air 13 2017, I woule like to ask if it has a built in anti virus? If none what is the best anti virus to purchase or download?
It has built in malware protection, but not virus protection. At this point there are not any Mac viruses in the wild, so unless you just want to help stop the spread of Windows viruses, you don't really need to run an AV setup in your Mac IMO.
I would check out the independent testing / review organization AV Comparatives.
Its still popular to believe that Macs don't need an AV program, but their popularity makes them a target and, even if you might not get attacked directly (or Apple releases a patch - sorry, 'update' - to cover a discovered vulnerability), I have no desire to pass along to someone else something in, say, a shared document or website.
AV Comparatives does tests on a regular basis, and have found several free AV ones that they believe do an excellent job - Avast, AVG and Avira among them. I'm currently running AVG. Check out the link and report here -