Ensuring that JavaScript code is cross-browser compatible means making sure that it works correctly and consistently across different web browsers such as Chrome, Firefox, Safari, Internet Explorer, Edge, etc. Here are a few best practices for writing cross-browser compatible JavaScript code:

1. Use modern JavaScript features: Modern JavaScript features like ES6 syntax, arrow functions, and destructuring assignment are not supported in older versions of some browsers. Therefore, it's important to use these features judiciously and provide fallbacks for older browsers that don't support them.

2. Test your code across multiple browsers: It's essential to test your JavaScript code across multiple browsers to ensure that it works correctly and consistently across all of them. This can be done manually or using automated testing tools like BrowserStack, Sauce Labs, etc.

3. Use feature detection: Instead of relying on the version of a browser to determine whether to use a particular JavaScript feature, you should use feature detection to check if a particular feature is supported by the browser. This approach ensures that your code works correctly even in browsers that don't support modern features.

4. Avoid using vendor-specific prefixes: Vendor-specific prefixes like -moz-, -webkit-, and -o- are used to add compatibility for specific browsers. However, these prefixes can be cumbersome to manage and can make your code harder to read and maintain. Instead, use modern JavaScript features that have broad browser support.

5. Use a code linter: A code linter is a tool that analyzes your JavaScript code to identify potential errors and issues that could cause problems in different browsers. By using a code linter, you can catch issues early on and ensure that your code is cross-browser compatible from the start.

6. Use polyfills: Polyfills are small pieces of JavaScript code that add support for modern features to older browsers. If there's a feature that you need to use but it's not supported by an older browser, consider using a polyfill to ensure cross-browser compatibility.

By following these best practices and testing our JavaScript code across multiple browsers, we can ensure that our code is cross-browser compatible and will work correctly on all modern web browsers.