This Gist shows you how to integrate with the AppTitleBar Back button for Windows 10 JavaScript Apps.

SystemBackButton.js

// Hardcoded Navigation Stack
var navigationStack = [
    { "url": "http://www.bing.com" },
    { "url": "http://www.msn.com" }
]

// GoBack function
function goBack() {
    var place = navigationStack.pop();
    // TODO: Navigate to place.url;
    setNavigationState();
}

// Function to set Navigation State
function setNavigationState() {
    if (typeof Windows !== 'undefined' &&
            typeof Windows.UI !== 'undefined' &&
            typeof Windows.UI.Core !== 'undefined') {
        var systemNavigation = Windows.UI.Core.SystemNavigationManager.getForCurrentView();

        if (navigationStack.length > 0) {
            systemNavigation.appViewBackButtonVisibility = Windows.UI.Core.AppViewBackButtonVisibility.visible;
        } else {
            systemNavigation.appViewBackButtonVisibility = Windows.UI.Core.AppViewBackButtonVisibility.collapsed;
        }
    }
}

// function to handle the system Navigation Event
function handleSystemNavigationEvent(args) {
    if (navigationStack.length <= 0) {
        return;
    }
    args.handled = true;
    goBack();
}

// Initialize the code on Windows load
addEventListener('load', function(){
     setNavigationState();
     if(typeof Windows !== 'undefined' &&
            typeof Windows.UI !== 'undefined' &&
            typeof Windows.UI.Core !== 'undefined') {

        var systemNavigationManager = Windows.UI.Core.SystemNavigationManager.getForCurrentView();
        systemNavigationManager.addEventListener("backrequested", this.handleSystemNavigationEvent.bind(this));
    }
});