History

chris 99d78c0f41 bank project updates		1 month ago
..
README.md	bank project updates	1 month ago
assignment.md	bank project updates	1 month ago
history.png	folder names	5 years ago

README.md

Unescape Escape

Build a Banking App Part 1: HTML Templates and Routes in a Web App

Modern web applications have revolutionized how we interact with digital services, from social media platforms to online banking systems. Unlike traditional websites that reload entire pages for each interaction, today's web apps provide seamless, app-like experiences that respond instantly to user actions. This smooth interaction is powered by sophisticated techniques that update only specific parts of the page, creating the fluid experience users expect.

Traditional Multi-Page Apps	Modern Single-Page Apps
Navigation	Full page reload for each screen
Performance	Slower due to complete HTML downloads
User Experience	Jarring page flashes
Data Sharing	Difficult between pages
Development	Multiple HTML files to maintain

Understanding the evolution:

Traditional apps require server requests for every navigation action
Modern SPAs load once and update content dynamically using JavaScript
User expectations now favor instant, seamless interactions
Performance benefits include reduced bandwidth and faster responses

In this lesson, you'll discover how to build the foundation of a single-page application (SPA) by creating a banking app with multiple screens that never require a full page reload. You'll learn how HTML templates work as reusable building blocks, how JavaScript routing enables navigation between different views, and how the browser's history API creates a natural browsing experience. These concepts form the backbone of modern web development and are used by popular frameworks like React, Vue, and Angular.

By the end of this lesson, you'll have built a functional multi-screen banking application that demonstrates professional web development techniques. You'll understand how to create smooth user experiences that rival native mobile apps, setting the foundation for more advanced web development skills. Let's start building something amazing!

Pre-Lecture Quiz

Pre-lecture quiz

Prerequisite

You need a local web server to test the web app we'll build in this lesson. If you don't have one, you can install Node.js and use the command npx lite-server from your project folder. It will create a local web server and open your app in a browser.

Preparation

On your computer, create a folder named bank with a file named index.html inside it. We'll start from this HTML boilerplate:

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Bank App</title>
  </head>
  <body>
    <!-- This is where you'll work -->
  </body>
</html>

Here's what this boilerplate provides:

Establishes the HTML5 document structure with proper DOCTYPE declaration
Configures character encoding as UTF-8 for international text support
Enables responsive design with the viewport meta tag for mobile compatibility
Sets a descriptive title that appears in the browser tab
Creates a clean body section where we'll build our application

📁 Project Structure Preview

By the end of this lesson, your project will contain:
bank/
├── index.html      
├── app.js          
└── style.css       
File responsibilities:

index.html: Contains all templates and provides the app structure

app.js: Handles routing, navigation, and template management

Templates: Define the UI for login, dashboard, and other screens

HTML Templates

HTML templates are one of the most powerful features for building dynamic web applications. Instead of creating separate HTML files for each screen in your app, templates allow you to define reusable HTML structures that can be dynamically loaded and displayed as needed. This approach significantly improves performance and provides a smoother user experience.

Think of templates as blueprints for different parts of your application. Just like an architect uses blueprints to construct different rooms in a building, you'll use HTML templates to construct different screens in your web app. The browser doesn't display these templates initially – they remain hidden until your JavaScript code activates them.

If you want to create multiple screens for a web page, one solution would be to create one HTML file for every screen you want to display. However, this solution comes with some inconvenience:

You have to reload the entire HTML when switching screen, which can be slow.
It's difficult to share data between the different screens.

Another approach is to have only one HTML file, and define multiple HTML templates using the <template> element. A template is a reusable HTML block that is not displayed by the browser, and needs to be instantiated at runtime using JavaScript.

Task

We'll create a bank app with two screens: the login page and the dashboard. First, let's add in the HTML body a placeholder element that we'll use to instantiate the different screens of our app:

<div id="app">Loading...</div>

Understanding this placeholder:

Creates a container with the ID "app" where all screens will be displayed
Shows a loading message until the JavaScript initializes the first screen
Provides a single mounting point for our dynamic content
Enables easy targeting from JavaScript using document.getElementById()

💡 Pro Tip: Since the content of this element will be replaced, we can put in a loading message or indicator that will be shown while the app is loading.

Next, let's add below the HTML template for the login page. For now we'll only put in there a title and a section containing a link that we'll use to perform the navigation.

<template id="login">
  <h1>Bank App</h1>
  <section>
    <a href="/dashboard">Login</a>
  </section>
</template>

Breaking down this login template:

Defines a template with the unique identifier "login" for JavaScript targeting
Includes a main heading that establishes the app's branding
Contains a semantic <section> element to group related content
Provides a navigation link that will route users to the dashboard

Then we'll add another HTML template for the dashboard page. This page will contain different sections:

A header with a title and a logout link
The current balance of the bank account
A list of transactions, displayed in a table

<template id="dashboard">
  <header>
    <h1>Bank App</h1>
    <a href="/login">Logout</a>
  </header>
  <section>
    Balance: 100$
  </section>
  <section>
    <h2>Transactions</h2>
    <table>
      <thead>
        <tr>
          <th>Date</th>
          <th>Object</th>
          <th>Amount</th>
        </tr>
      </thead>
      <tbody></tbody>
    </table>
  </section>
</template>

Let's understand each part of this dashboard:

Structures the page with a semantic <header> element containing navigation
Displays the app title consistently across screens for branding
Provides a logout link that routes back to the login screen
Shows the current account balance in a dedicated section
Organizes transaction data using a properly structured HTML table
Defines table headers for Date, Object, and Amount columns
Leaves the table body empty for dynamic content injection later

💡 Pro Tip: When creating HTML templates, if you want to see what it will look like, you can comment out the <template> and </template> lines by enclosing them with .

✅ Why do you think we use id attributes on the templates? Could we use something else like classes?

Displaying Templates with JavaScript

Now that we have our HTML templates defined, we need to bring them to life with JavaScript. Templates by themselves are invisible – they're like blueprints waiting to be built into actual structures. JavaScript provides the tools to select a template, create a working copy of it, and insert it into the visible part of your webpage.

The process of displaying a template involves three essential steps that work together to transform hidden template code into visible, interactive content. Understanding this process is crucial because it's the foundation of how modern web frameworks manage dynamic content.

If you try your current HTML file in a browser, you'll see that it gets stuck displaying Loading.... That's because we need to add some JavaScript code to instantiate and display the HTML templates.

Instantiating a template is usually done in 3 steps:

Retrieve the template element in the DOM, for example using document.getElementById.
Clone the template element, using cloneNode.
Attach it to the DOM under a visible element, for example using appendChild.

flowchart TD
    A[🔍 Step 1: Find Template] --> B[📋 Step 2: Clone Template]
    B --> C[🔗 Step 3: Attach to DOM]
    
    A1["document.getElementById('login')"] --> A
    B1["template.content.cloneNode(true)"] --> B  
    C1["app.appendChild(view)"] --> C
    
    C --> D[👁️ Template Visible to User]
    
    style A fill:#e1f5fe
    style B fill:#f3e5f5
    style C fill:#e8f5e8
    style D fill:#fff3e0

Visual breakdown of the process:

Step 1 locates the hidden template in the DOM structure
Step 2 creates a working copy that can be safely modified
Step 3 inserts the copy into the visible page area
Result is a functional screen that users can interact with

✅ Why do we need to clone the template before attaching it to the DOM? What do you think would happen if we skipped this step?

Task

Create a new file named app.js in your project folder and import that file in the <head> section of your HTML:

<script src="app.js" defer></script>

Understanding this script import:

Links the JavaScript file to our HTML document
Uses the defer attribute to ensure the script runs after HTML parsing completes
Enables access to all DOM elements since they're fully loaded before script execution
Follows modern best practices for script loading and performance

Now in app.js, we'll create a new function updateRoute:

function updateRoute(templateId) {
  const template = document.getElementById(templateId);
  const view = template.content.cloneNode(true);
  const app = document.getElementById('app');
  app.innerHTML = '';
  app.appendChild(view);
}

Step by step, here's what's happening:

Locates the template element using its unique ID
Creates a deep copy of the template's content using cloneNode(true)
Finds the app container where the content will be displayed
Clears any existing content from the app container
Inserts the cloned template content into the visible DOM

Now call this function with one of the template and look at the result.

updateRoute('login');

What this function call accomplishes:

Activates the login template by passing its ID as a parameter
Demonstrates how to programmatically switch between different app screens
Shows the login screen in place of the "Loading..." message

✅ What's the purpose of this code app.innerHTML = '';? What happens without it?

Creating Routes

Routing is the backbone of modern web applications – it's what makes your app feel like a cohesive experience rather than a collection of disconnected pages. When you navigate through a professional web app like Gmail or Twitter, the URL changes to reflect where you are, but the page doesn't reload. This seamless experience is powered by client-side routing, which maps URLs to specific content or views.

In traditional multi-page websites, the web server handles routing automatically by serving different HTML files based on the URL path. However, in single-page applications (SPAs), we need to implement this routing logic ourselves using JavaScript. This gives us complete control over how navigation works and enables the smooth, app-like experience users expect.

flowchart LR
    A["🌐 URL Path<br/>/dashboard"] --> B["🗺️ Routes Object<br/>Lookup"]
    B --> C["🎯 Template ID<br/>'dashboard'"]
    C --> D["📄 Find Template<br/>getElementById"]
    D --> E["👁️ Display Screen<br/>Clone & Append"]
    
    F["📍 /login"] --> G["🎯 'login'"]
    H["📍 /unknown"] --> I["❌ Not Found"]
    I --> J["🔄 Redirect to /login"]
    
    style B fill:#e3f2fd
    style E fill:#e8f5e8
    style I fill:#ffebee
    style J fill:#fff3e0

Understanding the routing flow:

URL changes trigger a lookup in our routes configuration
Valid routes map to specific template IDs for rendering
Invalid routes trigger fallback behavior to prevent broken states
Template rendering follows the three-step process we learned earlier

When talking about a web app, we call Routing the intent to map URLs to specific screens that should be displayed. On a website with multiple HTML files, this is done automatically as the file paths are reflected on the URL. For example, with these files in your project folder:

mywebsite/index.html
mywebsite/login.html
mywebsite/admin/index.html

If you create a web server with mywebsite as the root, the URL mapping will be:

https://site.com            --> mywebsite/index.html
https://site.com/login.html --> mywebsite/login.html
https://site.com/admin/     --> mywebsite/admin/index.html

However, for our web app we are using a single HTML file containing all the screens so this default behavior won't help us. We have to create this map manually and perform update the displayed template using JavaScript.

Task

We'll use a simple object to implement a map between URL paths and our templates. Add this object at the top of your app.js file.

const routes = {
  '/login': { templateId: 'login' },
  '/dashboard': { templateId: 'dashboard' },
};

Understanding this routes configuration:

Defines a mapping between URL paths and template identifiers
Uses object syntax where keys are URL paths and values contain template information
Enables easy lookup of which template to display for any given URL
Provides a scalable structure for adding new routes in the future

Now let's modify a bit the updateRoute function. Instead of passing directly the templateId as an argument, we want to retrieve it by first looking at the current URL, and then use our map to get the corresponding template ID value. We can use window.location.pathname to get only the path section from the URL.

function updateRoute() {
  const path = window.location.pathname;
  const route = routes[path];

  const template = document.getElementById(route.templateId);
  const view = template.content.cloneNode(true);
  const app = document.getElementById('app');
  app.innerHTML = '';
  app.appendChild(view);
}

Breaking down what happens here:

Extracts the current path from the browser's URL using window.location.pathname
Looks up the corresponding route configuration in our routes object
Retrieves the template ID from the route configuration
Follows the same template rendering process as before
Creates a dynamic system that responds to URL changes

Here we mapped the routes we declared to the corresponding template. You can try it that it works correctly by changing the URL manually in your browser.

✅ What happens if you enter an unknown path in the URL? How could we solve this?

Great routing logic is only half the equation – we also need intuitive navigation that allows users to move between screens effortlessly. In traditional websites, clicking a link causes the entire page to reload, which breaks the smooth experience we're trying to create. Instead, we'll implement programmatic navigation that updates the URL and content seamlessly.

Effective navigation in a single-page application requires coordinating two essential actions: updating the browser's URL to reflect the current screen, and displaying the appropriate content. This coordination ensures that users can bookmark specific screens, use the browser's back button, and share URLs that work exactly as expected.

🏗️ Architecture Insight: Navigation System Components

What you're building:

🔄 URL Management: Updates browser address bar without page reloads

📋 Template System: Swaps content dynamically based on current route

📚 History Integration: Maintains browser back/forward button functionality

🛡️ Error Handling: Graceful fallbacks for invalid or missing routes

How components work together:

Listens for navigation events (clicks, history changes)

Updates the URL using the History API

Renders the appropriate template for the new route

Maintains a seamless user experience throughout

The next step for our app is to add the possibility to navigate between pages without having to change the URL manually. This implies two things:

Updating the current URL
Updating the displayed template based on the new URL

We already took care of the second part with the updateRoute function, so we have to figure out how to update the current URL.

We'll have to use JavaScript and more specifically the history.pushState that allows to update the URL and create a new entry in the browsing history, without reloading the HTML.

⚠️ Important Note: While the HTML anchor element <a href> can be used on its own to create hyperlinks to different URLs, it will make the browser reload the HTML by default. It is necessary to prevent this behavior when handling routing with custom javascript, using the preventDefault() function on the click event.

Task

Let's create a new function we can use to navigate in our app:

function navigate(path) {
  window.history.pushState({}, path, path);
  updateRoute();
}

Understanding this navigation function:

Updates the browser's URL to the new path using history.pushState
Adds a new entry to the browser's history stack for proper back/forward button support
Triggers the updateRoute() function to display the corresponding template
Maintains the single-page app experience without page reloads

This method first updates the current URL based on the path given, then updates the template. The property window.location.origin returns the URL root, allowing us to reconstruct a complete URL from a given path.

Now that we have this function, we can take care of the problem we have if a path does not match any defined route. We'll modify the updateRoute function by adding a fallback to one of the existing route if we can't find a match.

function updateRoute() {
  const path = window.location.pathname;
  const route = routes[path];

  if (!route) {
    return navigate('/login');
  }

  const template = document.getElementById(route.templateId);
  const view = template.content.cloneNode(true);
  const app = document.getElementById('app');
  app.innerHTML = '';
  app.appendChild(view);
}

Key points to remember:

Checks if a route exists for the current path
Redirects to the login page when an invalid route is accessed
Provides a fallback mechanism that prevents broken navigation
Ensures users always see a valid screen, even with incorrect URLs

If a route cannot be found, we'll now redirect to the login page.

Now let's create a function to get the URL when a link is clicked, and to prevent the browser's default link behavior:

function onLinkClick(event) {
  event.preventDefault();
  navigate(event.target.href);
}

Breaking down this click handler:

Prevents the browser's default link behavior using preventDefault()
Extracts the destination URL from the clicked link element
Calls our custom navigate function instead of reloading the page
Maintains the smooth single-page application experience

<a href="/dashboard" onclick="onLinkClick(event)">Login</a>
...
<a href="/login" onclick="onLinkClick(event)">Logout</a>

What this onclick binding accomplishes:

Connects each link to our custom navigation system
Passes the click event to our onLinkClick function for processing
Enables smooth navigation without page reloads
Maintains proper URL structure that users can bookmark or share

The onclick attribute bind the click event to JavaScript code, here the call to the navigate() function.

Try clicking on these links, you should be now able to navigate between the different screens of your app.

✅ The history.pushState method is part of the HTML5 standard and implemented in all modern browsers. If you're building a web app for older browsers, there's a trick you can use in place of this API: using a hash (#) before the path you can implement routing that works with regular anchor navigation and does not reload the page, as it's purpose was to create internal links within a page.

Handling the Browser's Back and Forward Buttons

Users expect web applications to behave like traditional websites when it comes to browser navigation. When someone clicks the back button, they should return to the previous screen, and the forward button should work just as intuitively. However, single-page applications need special handling to make this work correctly.

The browser's navigation buttons interact with the history stack that we've been building with history.pushState. Each time we navigate to a new screen, we add an entry to this stack. When users click back or forward, the browser moves through this stack, but our application needs to respond to these changes and update the displayed content accordingly.

sequenceDiagram
    participant User
    participant Browser
    participant App
    participant Template
    
    User->>Browser: Clicks "Login" link
    Browser->>App: onclick event triggered
    App->>App: preventDefault() & navigate('/dashboard')
    App->>Browser: history.pushState('/dashboard')
    Browser->>Browser: URL updates to /dashboard
    App->>App: updateRoute() called
    App->>Template: Find & clone dashboard template
    Template->>App: Return cloned content
    App->>Browser: Replace app content with template
    Browser->>User: Display dashboard screen
    
    Note over User,Template: User clicks browser back button
    
    User->>Browser: Clicks back button
    Browser->>Browser: History moves back to /login
    Browser->>App: popstate event fired
    App->>App: updateRoute() called automatically
    App->>Template: Find & clone login template
    Template->>App: Return cloned content
    App->>Browser: Replace app content with template
    Browser->>User: Display login screen

Key interaction points:

User actions trigger navigation through clicks or browser buttons
App intercepts link clicks to prevent page reloads
History API manages URL changes and browser history stack
Templates provide the content structure for each screen
Event listeners ensure the app responds to all navigation types

Using the history.pushState creates new entries in the browser's navigation history. You can check that by holding the back button of your browser, it should display something like this:

If you try clicking on the back button a few times, you'll see that the current URL changes and the history is updated, but the same template keeps being displayed.

That's because the application does not know that we need to call updateRoute() every time the history changes. If you take a look at the history.pushState documentation, you can see that if the state changes - meaning that we moved to a different URL - the popstate event is triggered. We'll use that to fix that issue.

Task

To make sure the displayed template is updated when the browser history changes, we'll attach a new function that calls updateRoute(). We'll do that at the bottom of our app.js file:

window.onpopstate = () => updateRoute();
updateRoute();

Understanding this history integration:

Listens for popstate events that occur when users navigate with browser buttons
Uses an arrow function for concise event handler syntax
Calls updateRoute() automatically whenever the history state changes
Initializes the app by calling updateRoute() when the page first loads
Ensures the correct template displays regardless of how users navigate

💡 Pro Tip: We used an arrow function here to declare our popstate event handler for conciseness, but a regular function would work the same.

Here's a refresher video on arrow functions:

🎥 Click the image above for a video about arrow functions.

Now try to use the back and forward buttons of your browsers, and check that the displayed route is correctly updated this time.

GitHub Copilot Agent Challenge 🚀

Use the Agent mode to complete the following challenge:

Description: Enhance the banking app by implementing error handling and a 404 page template for invalid routes, improving the user experience when navigating to non-existent pages.

Prompt: Create a new HTML template with id "not-found" that displays a user-friendly 404 error page with styling. Then modify the JavaScript routing logic to show this template when users navigate to invalid URLs, and add a "Go Home" button that navigates back to the login page.

🚀 Challenge

Add a new template and route for a third page that shows the credits for this app.

Challenge goals:

Create a new HTML template with appropriate content structure
Add the new route to your routes configuration object
Include navigation links to and from the credits page
Test that all navigation works correctly with browser history

Post-Lecture Quiz

Post-lecture quiz

Review & Self Study

Routing is one of the surprisingly tricky parts of web development, especially as the web moves from page refresh behaviors to Single Page Application page refreshes. Read a little about how the Azure Static Web App service handles routing. Can you explain why some of the decisions described on that document are necessary?

Additional learning resources:

Explore how popular frameworks like React Router and Vue Router implement client-side routing
Research the differences between hash-based routing and history API routing
Learn about server-side rendering (SSR) and how it affects routing strategies
Investigate how Progressive Web Apps (PWAs) handle routing and navigation

Assignment

Improve the routing

README.md Unescape Escape

Build a Banking App Part 1: HTML Templates and Routes in a Web App

Pre-Lecture Quiz

Prerequisite

Preparation

HTML Templates

Task

Displaying Templates with JavaScript

Task

Creating Routes

Task

Adding Navigation

Task

Handling the Browser's Back and Forward Buttons

Task

GitHub Copilot Agent Challenge 🚀

🚀 Challenge

Post-Lecture Quiz

Review & Self Study

Assignment

README.md

Unescape Escape