Understanding `window.close` and `self.close` in Modern Browsers

Introduction

When developing web applications, controlling browser windows through JavaScript is often a desired feature. The window.close() method allows scripts to close the current window or tab programmatically. However, its behavior varies across different browsers due to security considerations. This tutorial delves into how and why modern browsers like Chrome and Firefox restrict the use of window.close(), along with potential workarounds and best practices.

How `window.close()` Works

The window.close() method is intended to close the browser window or tab in which it’s executed. According to the W3C specification, this method should only be permitted under certain conditions:

Script-Closable Windows: A window can be closed by script if it was opened using JavaScript via window.open(). This means the script has a handle on the window and closes one of its own creation.
User-Initiated Actions: The action to close must generally be initiated by the user, not automatically triggered without user interaction.

Security Implications

To prevent malicious scripts from disrupting users’ browsing sessions, browsers enforce strict rules regarding window.close(). These restrictions ensure that scripts cannot arbitrarily close windows or tabs that were not opened by them. This is a security measure to protect against unwanted disruption and potential phishing attacks where a script might close the window containing critical information.

Browser-Specific Behavior

Chrome

Chrome allows closing of windows only if they are "script-closable" as per its security model:

Script-Created Windows: If a window was opened using window.open(), it can be closed by scripts.
User Scripts and Extensions: In cases involving user scripts (like those run via Tampermonkey), you might notice that even script-created windows cannot always be closed due to additional security layers.

Firefox

Firefox follows similar rules but tends to enforce them more strictly:

Strict Enforcement: The method is allowed only for windows opened by a script using window.open(). Attempts to close other windows result in errors such as "Scripts may not close windows that were not opened by script."

Workarounds and Solutions

Making Use of Extensions

One effective way to reliably close windows across all browsers is by developing browser extensions. Extensions can interact with tabs and windows using APIs provided by the browser, offering more control than scripts run in web pages.

Chrome Extension API: Utilizes chrome.windows for managing windows.
```
chrome.windows.remove(windowId);
```

Scripting Techniques

User-Initiated Closure: Ensure that any window closing operation is initiated by a user action, like clicking a button. This can sometimes bypass restrictions as it aligns with the security model expecting user consent.
Self-Redirection Workaround:
- Previously used in Chrome to close windows opened via window.open(), this involves opening the current location and closing it immediately.
```
open(location, '_self').close();
```
Using User Scripts with Grants: For userscripts, specific grants can sometimes allow window closure.

Security Settings

While not recommended for general use due to security risks, altering browser settings like allow_scripts_to_close_windows in Firefox may enable script-based closing but at the cost of increased vulnerability.

Best Practices

Use Extensions: When reliable window control is required, consider developing an extension.
User Actions: Always base window-closing operations on user interactions to comply with security norms.
Clear Documentation: Ensure your code and any instructions are well-documented for maintainability and compliance with evolving browser standards.

Conclusion

Understanding the limitations of window.close() across different browsers is crucial for developing secure and functional web applications. By adhering to best practices and utilizing appropriate APIs, developers can achieve their goals while maintaining user trust and security.