Display a popup (with id="popup_expiredpage") when a page has expired

page_expiry.js

const wb_expiry = new function(){
	
    self.load_time = 99999999999999; //set automatically
    self.page_expiry = 60; //minutes
    self.reload_timer = {};
	
    this.hidden = 'hidden';
    this.vis_change = 'visibilitychange';
    this.hidden_state = 'visibilityState';
    this.currently_visible = true;
    
    this.init = () => {
        self.load_time = new Date().getTime();
        
        this.set_page_visibility_prefixes();
        document.addEventListener(this.vis_change, this.visibility_change);
        
        self.reload_timer = setInterval( self.check_time, 60000 ); // once per minute
    };
	
    this.set_page_visibility_prefixes = () => {
        if (typeof document.mozHidden !== "undefined") {
            this.hidden = "mozHidden";
            this.vis_change = "mozvisibilitychange";
            this.hidden_state = "mozVisibilityState";
        } else if (typeof document.msHidden !== "undefined") {
            this.hidden = "msHidden";
            this.vis_change = "msvisibilitychange";
            this.hidden_state = "msVisibilityState";
        } else if (typeof document.webkitHidden !== "undefined") {
            this.hidden = "webkitHidden";
            this.vis_change = "webkitvisibilitychange";
            this.hidden_state = "webkitVisibilityState";
        }
    };
    
    this.visibility_change = (e) => {
        if( this.hidden == document[ this.hidden_state ] || 'hidden' == document[ this.hidden_state ] ){
            this.currently_visible = false;
        } else {
            this.currently_visible = true;
            
            self.check_time();
        }
    };
    
    self.check_time = function(){
        const cur_time = new Date().getTime();
        
        console.log('Checking page time: ', cur_time - self.load_time, '<', self.page_expiry * 60 * 1000 );
        if( cur_time - self.load_time > self.page_expiry * 60 * 1000 ){
            console.log(' - Page expired. Prompting user to reload');
            clearInterval( self.reload_timer );
            
            // Show popup over the screen
            document.documentElement.classList.add('noscroll');
            const reloadPopup = document.querySelector('#popup_expiredpage');
            reloadPopup.style.display = "block";
            
            window.location.reload();
        } else {
            console.log(' - Ok.');
        }
    };
	
	
    // see if DOM is already available
    if (document.readyState === "complete" || document.readyState === "interactive") {
        // call on next available tick
        setTimeout(this.init, 1);
    } else {
        document.addEventListener("DOMContentLoaded", this.init);
    }
};