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";`