JavaScript usage in Visualforce pages and Lightning Components has distinct requirements:
1. **Visualforce Pages**:
- JavaScript can be included using `<apex:includeScript>` to reference scripts stored in static resources.
- Scripts should be loaded from static resources rather than third-party sources to avoid vulnerabilities.
- Directly loading JavaScript from third-party sources (e.g., CDNs) is insecure. Scripts should be stored in static resources and referenced using `$Resource`.
- HTML in merge fields is auto-encoded unless `escape="false"` is used. If disabled, developers must ensure proper encoding in the controller.
- Functions like `JSENCODE`, `JSINHTMLENCODE`, `HTMLENCODE`, and `URLENCODE` are available for secure data handling.
- Using `REQUIRESCRIPT` in managed packages is a security vulnerability as it injects code into the Salesforce origin.
- JavaScript is sandboxed in unique, vendor-specific origins.
2. **Lightning Components**:
- JavaScript must be sandboxed within the component's namespace using LockerService for isolation.
- Scripts must be included as static resources and wrapped in the Locker layer.
- Directly loading third-party JavaScript files from external sources is not allowed.
- Developers must sanitize user-controlled data before binding it to attributes or rendering it in the DOM, as the framework does not auto-encode HTML.
- Unsafe attributes like `href` or `iframe src` require additional sanitization to prevent vulnerabilities.
- Lightning components must follow strict Content Security Policy (CSP) guidelines, which block unsafe inline scripts.
- JavaScript is sandboxed in unique, vendor-specific lockers.
- LockerService is available for apps written for API version 40 or later.
Both frameworks emphasize using static resources and secure coding practices to mitigate risks.