app.js

JavaScript source file Reference

A .js file contains JavaScript source code. Web browsers run JavaScript to make pages interactive. Runtimes such as Node.js run the same language outside the browser. On Windows, the Windows Script Host can also run .js files. The content is plain text.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.