#! (pronounced shebang) tells the system what interpreter to pass that file to for execution. When a file has the above line is included, the system (*nix) will use Node.js to execute the file. This only works because Node makes a special exception for shebang characters. This will not work in a browser.
example file without .js extension
// hello #!/usr/bin/env node console.log('hello node env');
$ chmod +x ./hello $ ./hello $ hello node env