Here’s a simple way to get a URL parameter with JavaScript:

const urlParams = new URLSearchParams(window.location.search);

if (urlParams.get('lang') === 'en') {
	// Do something.
}

All URL parameters found are available under urlParams.

We can check a specific parameter using the .get method.

For example, urlParams.get('lang'). This returns the value of the lang parameter if it exists, and null if not.

If we don’t care about the value of a parameter and we just need to know if it exists in the URL, we can use the .has method.

For example, urlParams.has('lang'). This returns a boolean value: true if the lang parameter exists and false if not.

Other methods can be found in MDN: URLSearchParams - Methods.