JSFiddle - React, Tailwind, and code Playground

HTML

<strong>Index:<strong>
<input type="text" id="input" size="3"/>
<input type="button" id="moveLeft" value="Move Left" />
<input type="text" id="output" />

JavaScript

Object.prototype.getByIndex = function (property, index) {
    var n = 0;
    if (this.hasOwnProperty(property)) {
        for (var prop in this[property]) {
            if (n == index) {
                console.log('Found Match: ' + prop + ':' + this[property][prop]);
                return this[property][prop];
            } else {
                n++;
            }
        }
    }
    return undefined;
}

$('#moveLeft').on('click', function (nr) {
    var theBody = {
        bodies: {
            a: 'foo1',
            b: 'foo2',
            c: 'foo3'
        }
    };
    var index = parseInt($('#input').val());
    var currIndex = !isNaN(index) ? index : 0;
    var newBody = theBody.getByIndex('bodies', currIndex);
    $('#output').val(newBody);
});