JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://raw.github.com/douglascrockford/JSON-js/master/json2.js"></script>

JavaScript

var jsonData, obj, lines, keys, keyICareAbout, info, firstline;

// Note: json2.js is included from: https://github.com/douglascrockford/JSON-js

// If you require support from older browsers, do this:
if(!Object.keys) Object.keys = function(o){
 if (o !== Object(o))
      throw new TypeError('Object.keys called on non-object');
 var ret=[],p;
 for(p in o) if(Object.prototype.hasOwnProperty.call(o,p)) ret.push(p);
 return ret;
}

// The slashes escape the newlines, ignore them
jsonData = '{ \
    "Files": { \
             "Lines": { \
                    "198": { \
                           "firstline": "sometext" \
                    } \
             } \
    } \
}';

obj = JSON.parse(jsonData);
lines = obj.Files.Lines;
keys = Object.keys(lines);
keyICareAbout = keys[0];
info = lines[keyICareAbout];
firstline = info.firstline;

alert("My key: " + keyICareAbout + "\n" +
      "My info: " + JSON.stringify(info) + "\n" +
      "firstline: " + firstline);