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What is package.json file in ReactJS application? What is the use of it exactly? 

The package.json file in a ReactJS application is like the main information file for your project. It tells you (and the computer) important details such as:

1: The name and version of your app

2: Which libraries or packages your app needs to work (called dependencies)

3: Scripts to run tasks (like starting the app or running tests)

4: Other settings and metadata

In simple words:

package.json helps manage everything your React app needs. It makes it easy to install, update, and run your project. Without it, your app wouldn’t know what it needs or how to work properly.