yomichan/ext/bg/js/page-exit-prevention.js

/*
 * Copyright (C) 2019-2020  Alex Yatskov <alex@foosoft.net>
 * Author: Alex Yatskov <alex@foosoft.net>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */


class PageExitPrevention {
    constructor() {
    }

    start() {
        PageExitPrevention._addInstance(this);
    }

    end() {
        PageExitPrevention._removeInstance(this);
    }

    static _addInstance(instance) {
        const size = PageExitPrevention._instances.size;
        PageExitPrevention._instances.set(instance, true);
        if (size === 0) {
            window.addEventListener('beforeunload', PageExitPrevention._onBeforeUnload);
        }
    }

    static _removeInstance(instance) {
        if (
            PageExitPrevention._instances.delete(instance) &&
            PageExitPrevention._instances.size === 0
        ) {
            window.removeEventListener('beforeunload', PageExitPrevention._onBeforeUnload);
        }
    }

    static _onBeforeUnload(e) {
        if (PageExitPrevention._instances.size === 0) {
            return;
        }

        e.preventDefault();
        e.returnValue = '';
        return '';
    }
}

PageExitPrevention._instances = new Map();