If you wrote your code on your desktop and need to transfer it to your Raspberry Pi, there are a couple of ways to go about it: you can use rsync on an macOS X or Linux machine:
rsync ./blink-led.js
[email protected]:~/<project folder>/
Replace <project folder> with the folder you want to transfer into (for example, the book folder would be hands-on-robotics-with-javascript/ch2/blink-led).
For Windows, follow the guide for installing and using WinSCP at https://winscp.net/eng/docs/ui_commander.