before-unload

A generic onbeforeunload handler. Will check a condition, or a list of supplied conditions, to determine if it is safe to unload.

Take a look at the examples.

Installation

Install it from npm:

$ npm install before-unload

Usage

Simple condition

In the following example, the user will be prompted on the beforeunload event, if the ViewModel.hasChanges method returns a truthy value.

new BeforeUnload('Are you sure you want to leave?', function () {
    return ViewModel.hasChanges();
});

Multiple conditions

In the following example, the user will be prompted on the beforeunload event, if the ViewModel.hasChanges method or the Storage.unsavedChanges method returns a truthy value.

new BeforeUnload('Are you sure you want to leave?', [
    function () {
        return ViewModel.hasChanges();
    },
    function () {
        return Storage.unsavedChanges();
    }
]);

Conditions with custom messages

In the following example, a different warning is shown for one of the conditions.

new BeforeUnload('Are you sure you want to leave?', [
    function () {
        return ViewModel.hasChanges();
    },
    function () {
        return Storage.unsavedChanges();
    },
    function () {
        return SomeObject.isItOkayToLeave() ? false : 'A custom error message';
    }
]);

• If a false is returned from a condition, no warning is shown.
• If a boolean true value is returned, the default message set in the constructor is shown.
• If a string value is returned, that is displayed.

Unregistering the beforeunload event listener.

If you store a reference to the BeforeUnload object, you can unregister the event listener.

If a user attempts to leave the page after the unregister call, she will not be prompted.

var beforeUnload = new BeforeUnload(
    'Are you sure you want to leave?',
    function () {}
);

beforeUnload.unregister();