How do I warn a user of unsaved changes before leaving a page in Vue

Assuming you’re using vue-router (and you probably should be), then you’ll want to use the beforeRouteLeave guard. The documentation even gives an example of this exact situation:

beforeRouteLeave (to, from , next) {
  const answer = window.confirm('Do you really want to leave? you have unsaved changes!')
  if (answer) {
    next()
  } else {
    next(false)
  }
}

That can be added directly right on your component:

components: { ... },
mounted: { ... }
methods: { ... },
beforeRouteLeave (to, from, next) { ... }