Both Chrome and Firefox have excellent JavaScript consoles, also known as DevTools, for debugging your JavaScript.
You can find Developer tools in your Chrome's menu or Web Console in FireFox's menu. If you're using a different browser, or a mobile phone, we strongly recommend switching to desktop Firefox or Chrome.
You can also use https://repl.it/ to run code online.
This is how you print to the console: console.log('Hello world!')