What is a shebang line, and why does it need to be the first line of an executable Node.js CLI script?

A shebang line (#!/usr/bin/env node) tells Unix-like operating systems which interpreter should execute the file when it's run directly as a program (like ./my-script.js) rather than being explicitly invoked via 'node my-script.js' -- using 'env node' rather than hardcoding a specific path like /usr/bin/node makes the script portable across systems where Node.js might be installed in different locations, since env looks up 'node' in the user's PATH.

#!/usr/bin/env node
console.log('Hello from a CLI tool!');

// After making it executable: chmod +x my-script.js
// It can then be run directly: ./my-script.js