JavaScript in Construct works the same as JavaScript in a web page. You can write the same code you'd use on a web page. There's nothing special about Construct's "JS implementation" - it's not Construct that implements JS, it's the browser.
You may have difficulty with some APIs when Construct runs in a web worker, but by default when you add JS code it runs in the DOM, so as long as you don't change the 'Use worker' setting then there's nothing different to coding a web page.