Here's a good reference on the topic html5rocks.com/en/tutorials/workers/basics

Web workers provide a kind of multithreading, you use them to run stuff in the background without affecting your users experience.

• One way workers are useful is to allow your code to perform processor-intensive calculations without blocking the user interface thread.
• You can use web workers to poll urls in the background.
• You can even use web workers to render games in the background and display it when the user needs it.