Service Worker

The Service worker is a script that runs on the browser. So whenever there is an out bound request to the server from the browser the service worker checks a cache history of the request and deals with the request by itself using the cache history. The service worker runs even when a user closes the tap meaning that it can recognize the next request form the users when they visit again. 

 

This allows a fast loading time of the website that improves user experience as it can handle any request by itself without sending it to the server using the cache history (works similar as network proxy).

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참고

Angular