title: How to show confirmation of unsaved changes
tags: web js

From [this stackoverflow](https://stackoverflow.com/questions/10311341/confirmation-before-closing-of-tab-browser)
```js
function onBeforeUnload(e) {
    if (thereAreUnsavedChanges()) {
        e.preventDefault();
        e.returnValue = '';
        return;
    }

    delete e['returnValue'];
}

window.addEventListener('beforeunload', onBeforeUnload);
```