sits beween backend and frontend. → When HTTP requests reach the backend the JSON files are parsed and available.

Transfers data.

Executes code that can manipulate request and response objects for every request-response cycle.

Environment variables provide a standard way for configurability .

Separating configuration (shouldn't always be seen from the outside) from code.

There are also several other ways to pass configuration to the program (pass parameters, read from configuration file, etc.)