McDonald's nationwide are giving away free french fries every Friday for the rest of 2018, but of course, there's a catch.

You can get the free french fries at McDonald's every Friday in 2018 if you spend at least $1 on the McDonald's mobile app. You can only get that one free order on that given day.

McDonald's has been trying to appeal more to its younger customers with the app promotion in order to revitalize recent declining sales. McDonald's saves on labor costs when their customers order digitally through the app.

If you're wondering, 'What can I get for $1 at McDonald's?' The fairly new '$1 $2 $3 Dollar Menu' has a few items that can go with your free order of fries you'll receive. A  cheeseburger, McChicken sandwich, sausage burrito, or any size soft drink are all just $1. Order one of those or anything that cost more than $1 through the app and you'll get an order of free fries.

The promotion began on National French Fry Day (July 13th) and is said to continue through December 30th (the last Friday of 2018 is December 28th).