AgentScript runs in the browser using the JavaScript language which has a fascinating history, having been introduced for the Netscape server in the mid 1990's. Here's a JavaScript TimeLine
There are LOTS of resources for learning JavaScript. You can find several books, too many to list here!
And there are several tutorials and videos to use, just google javascript tutorial for them.
JavaScript in 14 Minutes
One of the best for the absolute beginner is JavaScript in 14 Minutes. It uses the browser's developer console. It will detect your OS and default browseer. I recommend Chrome but it supports them all.
And here's the Developer Console
It gives you small snippets of code which you cut/paste into the console. here's one:
Eloquent Javascript
Here is a much loved book: https://eloquentjavascript.net/
This is a complete, easy to use reference, "Modern JavaScript": https://javascript.info/
MDN
And the Mozilla Developer Network (fMDN) https://developer.mozilla.org/
is the Source of All Truth for all things browser. The JavaScript section https://developer.mozilla.org/en-US/docs/Web/JavaScript is quite good, and you can search for any JavaScript primitive easily.
Developer Console
console.log('you got executed from an markdown file')