Pop-up to alert users

I have a web page with an order form. The problem is that some people complete the required units, and then insert of pressing the "Add to Cart" button, they click on other links on my page, or press the back button, and the info is lost (the units entered for each product). Is there a way of displaying a warning message like "You have some units to add to your basket. Please click the Add button", when the user has added some information in my text boxes (all of them are named basket_units_1, basket_units_2, etc when the number at the end is the product_id). This event should be happening only when the user has added some units in my textboxes, and presses anything else other than the Add button. Of course, the event should be cancelled as well

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