Progressive Web App (PWA)

Web application can be configured as a Progressive Web App (PWA). Here's a brief explanation of why this happens:

1. Progressive Web App (PWA) features: React.js applications can be configured as PWAs, which allow web apps to behave more like native mobile apps. This includes features like offline functionality, push notifications, and the ability to be installed on the device's home screen.

2. Web App Manifest: To enable the "Install app" prompt, the React application must have a web app manifest file (usually named manifest.json). This file defines how the app should behave when installed on the device.

3. Service Worker: A service worker is typically implemented to handle caching and offline functionality, which is another requirement for PWAs.

4. HTTPS: The app must be served over HTTPS to be eligible for installation.

5. Engagement heuristics: Chrome has certain engagement heuristics that determine when to show the install prompt. These may include factors like how often the user visits the site, how long they spend on it, and other usage patterns.

6. User's previous actions: If a user has previously dismissed the install prompt, Chrome may wait before showing it again.

The "Install app" message appears because your React.js application meets the criteria for being a PWA, and Chrome has determined that it might be beneficial for the user to install it for a more app-like experience.