Observe property reads in JavaScript

observe.js

/* Recursively observe property reads of an object. 
 *
 * Takes the target and an object for returning results. Returns the proxied
 * target. Results are assigned to result.reads.
 *
 * Example:
 *
 *     var target = {a: 1};
 *     var result = {};
 *     target = observeReads(target, result);
 *     console.log(result.reads['root.a'])  // => undefined
 *     console.log(target.a);               // => 1
 *     console.log(result.reads['root.a'])  // => true
 *
 * Requires Proxy, which is part of ES6 but so far only implemented by Firefox
 * and Edge.
 */
function observeReads(target, result) {
  result.reads = {};

  function handler(prefix) {
    var cache = {};

    return {
      get: function(target, property) {
        var key = prefix + '.' + property;
        var value = target[property];
        result.reads[key] = true;
        if (value !== null && typeof value === 'object') {
          // We cache the proxy objects to ensure that foo.a === foo.a. If we
          // would always return a new Proxy(...), that wouldn't be the case.
          if (!(property in cache)) {
            cache[property] = new Proxy(value, handler(key));
          }
          return cache[property];
        } else {
          return value;
        }
      },
      set: function(target, property, value) {
        target[property] = value;
        delete cache[property];
        return true;
      }
    }
  }

  return new Proxy(target, handler('root'));
}