Wait Until September
If Apple plans to update the MacBook this year, this is the last time that would make sense. It's also when a Mac Pro update is coming.
Apple routinely announces new hardware in the fall, or right before it with a press release. There is a good chance that this is when we will see a new MacBook.
If you wait until then, you should still be able to find good deals on the 2017 model if they don't announce a 2019 MacBook, and if they do announce one then you get the benefits of buying newer hardware.
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