Black Friday sales are almost upon us, and it’s a foregone conclusion that Chromebooks will feature prominently in many retailers’ sales. Many of these computers are already a good value without any discounts, but Black Friday sales can make them worthy of an impulse purchase. However, you’d do well to make sure that laptop is worth buying even for a pittance before you pull the trigger.

One of the primary selling points of Chromebooks is how little you need to do to maintain them. You just log in with a Google account, go about your business in the Chrome browser, and know that Google will occasionally update the core components silently in the background. If you want to get fancy with Linux apps or the Play Store, you can do that as well. Google rolls out updated support for those features in new versions of Chrome OS, too.

Chrome OS updates are important not only for delivering new features, but they’re also vital for keeping your device safe. There’s nothing much living in Chrome OS to be hacked or infected by malware, but the Chrome browser could be exploited as you wander around the internet just like it could on Windows or macOS. When Chromebooks are supported, they get automatic updates and are always secure. Those updates don’t last forever, though.

Google guarantees at least five years of update support for Chromebooks, so there are already some older Chromebooks that are outside that support window. There are many more that are coming up on the end of support. These are devices you might see discounted to almost nothing as a way to grab your attention.

Before you buy any Chromebook this holiday season, there’s a quick and easy way to make sure it’s worth buying. Head to Google’s Chromebook update support page, and find the model number (it’s organized by vendor). You might see several devices with very similar names, but one has four more years of support and the other expired last summer. Maybe another only has another year and change of updates remaining.

You should consider those dates the expiration date of a laptop. If you see something on sale but it’s only got a year of updates left, just imagine you’re paying the asking price for a year of using that computer. Does it still seem like a good value? Then by all means, buy it.

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