Need to read Item Id value in HTML file opened by javascript Action method

I have created an  Action with Type='Item'. When I right click on the Item I want to read the item ID. I am able to read it in the action method say "showData()"(java script). In this java script method I am opening an HTML page. This html page has java script code i.e script section present in html file. I need to retrieve the value of item in this script section. Need suggestions on how to achieve this?

Note : I created a Form and opened that form in showData() method. Inside the form I have added an html element. this html code has javascript and CSS.

Appreciate help here.