1. beforeunloadevent api

  2. beforeunloadevent api: returnvalue

  3. beforeunloadevent api: user interaction required for dialog box

  4. window api: `beforeunload` event

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

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

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