beforeunload 이벤트는 윈도우, 문서 및 리소스가 언로드 될 때 발생합니다.  문서의 내용은 여전히 표시되며 이 시점에서 이벤트는 아직 취소 가능합니다.

만약 returnValue이벤트 프로퍼티에 문자열을 할당했다면, 유저에게 페이지를 나갈지 확인하는 대화상자에 나타납니다. 어떤 브라우저는 대화상자에 문자열이 보이지만, 다른 브라우저에서는 그들의 자체 메시지가 보입니다. 아무것도 할당하지 않는다면 이 이벤트는 대화상자 없이 조용히 진행됩니다.

Note: To combat unwanted pop-ups, browsers may not display prompts created in beforeunload event handlers unless the page has been interacted with, or may even not display them at all.

Attaching an event handler/listener to window or document's beforeunload event prevents browsers from using in-memory page navigation caches, like Firefox's Back-Forward cache or WebKit's Page Cache.

Bubbles No
Cancelable Yes
Target objects defaultView
Interface Event


Property Type Description
target Read only EventTarget The event target (the topmost target in the DOM tree).
type Read only DOMString The type of event.
bubbles Read only Boolean Does the event normally bubble?
cancelable Read only Boolean Is it possible to cancel the event?
returnValue DOMString The current return value of the event (the message to show the user).


window.addEventListener("beforeunload", function (event) {
  event.returnValue = "\\o/";

// is equivalent to
window.addEventListener("beforeunload", function (event) {


Since 25 May 2011, the HTML5 specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event. See the HTML5 specification for more details.

Various browsers ignore the result of the event and do not ask the user for confirmation at all. The document will always be unloaded automatically. Firefox has a switch named dom.disable_beforeunload in about:config to enable this behaviour.


Specification Status Comment
HTML Living Standard
The definition of 'beforeunload' in that specification.
Living Standard  
The definition of 'beforeunload' in that specification.
Recommendation Initial definition

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!
Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 1.0 (Yes) 1 4 12 3
Custom text support removed 51.0 No support 44.0 (44.0)   38 9.1
Feature Android Android Webview Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile Chrome for Android
Basic support ? (Yes) (Yes) ? ? ? (no) defect (Yes)
Custom text support removed ? 51.0 No support 44.0 (44.0)       51.0

See also