1. beforeunloadevent api

  2. beforeunloadevent api: returnvalue

  3. beforeunloadevent api: user interaction required for dialog box

  4. windoweventhandlers api: onbeforeunload

  5. window api: `beforeunload` event

  6. window api: `beforeunload` event: activation using `event.returnvalue = "string";`

  7. window api: `beforeunload` event: activation using `event.preventdefault()`

  8. window api: `beforeunload` event: activation using `return "string";`

  9. html element: body: onbeforeunload