设置并注入JScript,以便在传输后从www.example.com上的文件中显示拉出的信息

Looking to do a couple things. I'd like to create a Jscript file that will display content at the top of the screen. Above everything. The "content" being a specific section of data from a XHR-JSON that was transferred from the host. Also, the ability for the jscript to update the "content" every time the file is transferred.

Basically, Server sends a XHR JSON from "www.abc.com/1/2/ThisSectionVaries/FILE, jscript displays a certain line pulled from the middle of the data onto the top of the screen.

Where to start. I am not a coding pro, but I can figure my way around. Essentially , I need to create an API to capture certain information and a jscript to display it.

Before calling it a duplicate of Pull JSON API results from URL and display in PHP I am trying to go a step further than that, and preferably not PHP because Lord knows I have no idea how to use PHP.