JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = '{"ID":1234, "CONTENT":"HELLO"}';

var obj = $.parseJSON(str);
$.each(obj, function(i, v) {
    obj[i.toLowerCase()] = v;
    delete obj[i];
});

console.log(obj);
//{id: 1234, content: "HELLO"}