To trigger JavaScript from InDesign, create a new Hyperlink using the Hyperlinks panel (Window > Interactive).
Select URL from the Link To menu.
In the Destination, start with "javascript:" and then type the JavaScript code that you wish to execute.
In the example above, the code will trigger an alert in the browser with "test" as the text in the alert.
(alert is a built-in JavaScript. It's simply used for demonstration purposes in this article).
You can apply the same type of code as a button action via the Buttons & Forms panel (Window > Interactive).
To do so, add a Go To URL action to your button.
Then type "javascript:" and the code you'd like to execute.
You can attach a JavaScript file with custom functions in the Resources section of the in5 export dialog.
When you export with in5 (in5 > Export HTML5 with in5...), you'll send this in action.