Not only can you integrate JavaScript with Blazor, that integration provides a strategy for moving your existing pages to Blazor without having to rewrite your existing JavaScript code.

If you have an existing web application, open it in Visual Studio Code (preferably copying it to a new development branch) and then use the PWA Studio tools to convert it into a PWA.

An HTML Helper is a bit of Razor code that can be called from multiple places in a View. But, if you put your Helper in the right place, you can also use it from any View in your application.