beforeunloadevent api
beforeunloadevent api: returnvalue
beforeunloadevent api: user interaction required for dialog box
window api: `beforeunload` event
window api: `beforeunload` event: activation using `event.returnvalue = "string";`
window api: `beforeunload` event: activation using `event.preventdefault()`
window api: `beforeunload` event: activation using `return "string";`
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