If a user makes a change then tries to close their tab, window, or click a link to navigate away, they will see a default browser message warning them about unsaved changes. This message is different on each browser and cannot be edited.
The message will say something similar to:
You have unsaved changes, are you sure you want to leave?
Add the attribute
data-module="warn-on-unsaved-changes" to any form element. If that form is changed but not submitted, then the warning will be triggered when the user attempts to leave.
<form action="…" data-module="warn-on-unsaved-changes"> … </form>