getDeepValue.js

var setDeepValue = function(obj, deepProp, val) {

  var props = deepProp.split('.'),
      root = obj,
      i, n, p, t;

  for (i = 0, n = props.length - 1; i < n; ++i) {
    p = props[i];
    t = typeof obj[p];
    obj = obj[p] = (t === 'object' || t === 'function') ? obj[p] : {};
  }

  obj[props[i]] = val;

  return root;
};

// Example:
var foo = {};
setDeepValue(foo, 'deeply.nested.property', 'bar');

// Added return to allow building of new objects
var obj = setDeepValue({}, 'deeply.nested.property', 'bar');


/**
 * foo = {
 *   deeply: {
 *     nested: {
 *       property: 'bar'
 *     }
 *   }
 * }
 */

var getDeepValue = function(obj, deepProp) {

  var props = deepProp.split('.'),
      i, n;

  for (i = 0, n = props.length; i < n; ++i) {
    if (typeof (obj = obj[props[i]]) === 'undefined') {
      return;
    }
  }

  return obj;
};

// Example:
var val = getDeepValue(foo, 'deeply.nested.property'); // val = bar