The dependencies in your project's package.json allow the project to install the versions of the modules it depends on. In an npm package.json file, there are few types of dependencies: dependencies, devDependencies, and peerDependencies.

dependencies: These are the packages that are required for your application to run in production. They are installed when you run the npm install command, and are typically listed in the "dependencies" object in the package.json file. For example:

"dependencies": {

  "express": "^4.17.1",

  "mongoose": "^5.11.15"

}

Here, express and mongoose are both packages required for the application to run in production.

devDependencies: These are the packages that are only required during development and testing, such as testing frameworks or build tools. They are installed when you run the npm install --dev command, and are typically listed in the "devDependencies" object in the package.json file. For example:

"devDependencies": {

  "jest": "^26.6.3",

  "nodemon": "^2.0.7"

}

Here, jest and nodemon are only needed during development and testing.

peerDependencies: These are the packages that your module expects to be present in the consumer's environment, and are typically not installed automatically. They are listed in the "peerDependencies" object in the package.json file. For example:
"peerDependencies": {

  "react": "^17.0.1",

  "react-dom": "^17.0.1"

}


Here, the module expects the consumer to have react and react-dom installed in their environment, but does not install them automatically. Instead, it gives a warning message if they are not present in the consumer's environment.

It's important to understand the differences between these types of dependencies and to use them appropriately in your package.json file.