HTML5 realizes network state change detection through online/offline events. The specific methods are: 1. Use window.addEventListener to listen for 'online' and 'offline' events respectively; 2. Write a callback function to handle network state changes, such as prompting the user, pausing asynchronous requests or switching local cache mode; 3. You can dynamically display the network state with the navigator.onLine attribute and DOM element. Notes include: Events only reflect whether the browser is connected to the network rather than the server accessibility. Mobile devices may frequently trigger events that require anti-shake processing, and it is recommended to improve the accuracy of judgment based on heartbeat requests.

HTML5 provides online/offline events, which can be used to monitor changes in users' network status. This function is very useful in web applications that need to adjust behavior according to network connection conditions, such as reminding users that there is currently no network, pausing data synchronization, etc.

How to detect changes in network status

HTML5's window object provides two events: online and offline . These two events are triggered when the browser detects that a network connection status has changed.

You can add listeners in the following ways:

 window.addEventListener('online', function() {
  console.log('Network Recovered');
});

window.addEventListener('offline', function() {
  console.log('Network is disconnected');
});

These events are suitable for most modern browsers, including Chrome, Firefox, Edge, and more. However, it should be noted that they only reflect whether the browser can access the network, not whether the server is accessible.

Common usage scenarios and suggestions

• Prompt the user's network status : When the user is disconnected from the Internet, a prompt bar is displayed to tell them that they are currently unable to connect to the Internet.
• Pause asynchronous request : Pause the sending of AJAX request when the network is disconnected, and wait until the network is restored before continuing.
• Switch local cache mode : You can enable locally stored data offline to improve user experience.

For example, you can add a status indicator to the page:

 <div id="status"   style="max-width:90%"></div>

Then use JavaScript to control its display content:

 window.addEventListener('online', updateNetworkStatus);
window.addEventListener('offline', updateNetworkStatus);

function updateNetworkStatus() {
  const status = navigator.onLine ? 'online' : 'offline';
  const statusDiv = document.getElementById('status');
  statusDiv.textContent = 'Current status:' status;
  statusDiv.style.display = 'block';
}

Notes and limitations

Although the HTML5 online/offline event is very practical, there are several things to note:

• They rely on the browser's network detection mechanism and may not be accurate enough. For example, even if the browser thinks it is online, it may not be able to access your server.
• On mobile devices, network status may be switched frequently, resulting in events being triggered multiple times and requiring anti-shake processing.
• Some browser extensions or proxy settings may interfere with the judgment of network status.
• It is not supported to determine which website is unreachable, and it can only determine whether there is a network connection.

If your application has high requirements for judging network status, it is recommended to combine other methods, such as sending heartbeat requests to the server regularly to confirm connectivity.

summary

Basically that's it. HTML5's online/offline event is a lightweight and convenient tool suitable for basic network state awareness. Although it has some limitations, it is enough in many scenarios. The rational use of these events can significantly improve the robustness and user experience of web applications.

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