For testing things quickly in JavaScript, open the JavaScript Console in the Google Chrome browser. View menu > Developer > JavaScript Console.
You’ll see the console at the bottom of the current page.
The console gives you a command line so you can test short pieces of code to see what they do.
If you want to use the console as a stand-alone window, undock it with the button in the lower left corner.
You can keep the window open for quick access while you’re learning JavaScript.