get the form data object

Am i just blind, or can you not get the form data form an inertia form?

I have a simple form like this

  data() {
      form: this.$inertia.form({
        expenses: [],
        partner_expenses: [],
      })
    }
  },

I am wanting to grab that data and check it to see if has changed incase the user clicks away to go elsewhere before saving

I have the code for beforeunload and Inertia.on('before') loaded and working, now I just need to somehow get the data from the form to check to see if it has been updated.

How can I go about doing this?