How does Firebase differentiate whether it is deploying to hosting or cloud functions when using "firebase deploy"?

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Firebase differentiates between deploying to hosting or cloud functions by looking at the contents of the project's firebase.json file.

If the firebase.json file has a "hosting" property defined, then running the command firebase deploy will deploy the project to Firebase Hosting.

If the firebase.json file has a "functions" property defined, then running the command firebase deploy will deploy the project's Cloud Functions.

If the firebase.json file has both a "hosting" and "functions" property defined, then running the command firebase deploy without any additional parameters will deploy both Firebase Hosting and Cloud Functions.

However, if the firebase.json file only has a "database" property defined, running the command firebase deploy will not do anything as it does not have anything to deploy.