Edit in JSFiddle

var myButton = document.getElementById('my-button'),
		myResults = document.getElementById('my-results');

var myObject = {
	one: {
  	two: {
    	three: 'This is the string you want to see'
    }
  }
}


function getObjectProps(objTarget, strProp) {
	// objTarget: An object that is several levels deep.
  // strProp: Property string for that object.
  // Example: getObjectProps(myObject, "one.two.three");

// Reference to the current object, which will be redefined as we drill down into it.
  // Also making an array by splitting the string along the period.
  var objTemp = objTarget,
      arrProp = strProp.split('.');

	// Loop through the new array, and go deeper into the object each time.
  for (var i = 0; i < arrProp.length; i++) {
    objTemp = objTemp[arrProp[i]];
  }
  // Return the final object property.
  return objTemp;
};

myButton.addEventListener("click", function(){
  myResults.innerHTML = getObjectProps(myObject, "one.two.three");
}, false);

<button id="my-button">Check Result</button>
<p>Click to dig down into the "myObject" object to the value of the "three" property. This is meant for use with Underscore's "groupBy" method, when you're using an iterator function to sort by a deeper property.
</p>
<p><strong>The value is: </strong><span id="my-results"></span></p>

button {
  padding: .5rem;
  font-size: 1.5rem;
}