Back

 Türkçe

/ 4 min read

Efficiency in Web Development: Hot/Live Reloading

In today’s web development processes, waiting for the code we write to reflect in the browser can seriously decrease productivity. Fortunately, thanks to technologies like Hot Reloading and Live Reloading, these waiting times are minimized. So, what do these terms mean, how do they work, and why should we use them?

Live Reloading: Automatic Page Refresh

Live Reloading is a simple yet effective feature that many developers entering the web development world first encounter. Basically, when you make any changes to a project’s source code (HTML, CSS, or JavaScript) and save it, it ensures that your browser automatically refreshes the entire page.

How Does It Work?

The mechanism behind Live Reloading typically includes these steps:

  1. Development Server: In your development environment (for example, Webpack Dev Server, Browsersync, or a simple Node.js server), a development server runs. This server serves the project to the browser.
  2. File Watcher: The server actively monitors your project’s source code files (HTML, CSS, JavaScript) in the background. This monitoring is done through libraries that listen to the operating system’s file system events (like Watchdog in Python, Chokidar in JavaScript).
  3. Change Detection: When any change is detected in a file (after a save operation), the file watcher communicates this information to the development server.
  4. Signal Sending: The development server sends a “refresh page” signal to a special client script running in the browser (usually over a WebSocket connection). This client script is injected into your page as a JavaScript file provided by the development server.
  5. Browser Refresh: The client script that receives the signal triggers the browser’s location.reload() function, causing the entire page to reload.

When and Where Is It Used?

Live Reloading is ideal especially for simple websites, static pages, or projects where intensive work is done on CSS/HTML. It’s quite effective when refreshing the entire page on any change doesn’t create problems or when the page state is not complex. It comes by default in older or simple configurations of tools like Vue CLI, Create React App.

Hot Reloading (HMR): Instantly Replacing Changed Modules

Hot Reloading or more technically Hot Module Replacement (HMR), is a more advanced technology that goes one step beyond Live Reloading. The main difference is that when a code change is detected, instead of refreshing the entire page, it “patches” only the changed module (or component) into the current running state of the application. This is vitally important especially when developing SPAs (Single Page Applications).

How Does It Work?

The mechanism behind HMR is more complex, but basically includes:

  1. Development Server and Module Bundler Integration: HMR works in tight integration with development servers of modern module bundlers like Webpack, Vite, Parcel.
  2. File Monitoring and Change Detection: Like in Live Reloading, a file watcher detects source code changes.
  3. Module-Level Change Detection: The module bundler intelligently determines which piece of code has changed and which module or modules this change affects.
  4. Creating Update Packages: “Update bundles” are created for the changed modules. These packages contain only the content of the changed code.
  5. WebSocket/SSE Communication: The development server sends a notification to the HMR client in the browser over a WebSocket or Server-Sent Events (SSE) connection about which modules have been updated.
  6. Module Replacement in Browser:
    • When the HMR client receives this notification, it commands the download of the changed modules.
    • The downloaded new modules replace the old modules in the running application.
    • During this process, HMR typically triggers re-rendering of the changed component while the application’s current state is usually preserved. For example, if you’re filling out a form and make a CSS change, you can see the change instantly without the form being reset.
    • If the dependency chain becomes complex when a module changes or cannot be directly handled by HMR, a full page refresh may be done as a fallback.

When and Where Is It Used?

HMR is indispensable especially for complex single page applications (SPAs) developed with modern JavaScript frameworks like React, Vue, Angular. Being able to get instant feedback without losing the application’s state incredibly speeds up the development cycle and makes debugging easier. For example, seeing an instant style change in a payment component without losing products in a user’s cart is possible with HMR.

Why Should We Use Them?

Both Live Reloading and Hot Reloading offer fundamental benefits that transform your development process:

  • Speed and Efficiency: Eliminates the need for manual page refresh after code changes. This, especially with HMR, allows you to get feedback within seconds or even milliseconds, helping you maintain your “flow”.
  • Faster Feedback Loop: Speeds up debugging and iteration processes by instantly seeing the visual or functional effects of your code.
  • Developer Experience (DX): Provides an uninterrupted development flow. Significantly improves developer experience by eliminating the need to constantly switch to the browser and press the refresh button.
  • Application State Preservation (for HMR): Especially in complex SPAs, HMR saves time by preserving the application’s current state (user input, selected items, etc.) and reduces repetitive manual steps.