A Progressive Web App (PWA) is a combination of web and native applications providing such features as push notifications, offline working, installing on the homescreen, etc. Simply put, it is a technology that adds mobile app functionality to websites.
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The original nucleus of a PWA is Service Worker. It is a proxy layer between front-end and back-end placed in the browser and all the requests pass through it.
A PWA uses Hypertext Transfer Protocol Secure (HTTPS) which makes all website resources to be sent via bidirectional encryption. In fact, you can get an SSL certificate for free, but some hosting providers even receive them on their own.
The matter is that the app shell is stored on the client’s side and becomes uploaded during the PWA launch. Only then the dynamic data is uploaded to the app shell from a network.
It is the most valuable feature of Progressive Web Apps as it can increase engagement by approximately 4 times. For example, Treebo received a 4x increase in conversion rate after launching their PWA with push notifications.