function IntersectionObserver(callback, opt_options) {

  var options = opt_options || {};

  if (typeof callback != 'function') {
    throw new Error('callback must be a function');
  }

  if (options.root && options.root.nodeType != 1) {
    throw new Error('root must be an Element');
  }

  // Binds and throttles `this._checkForIntersections`.
  this._checkForIntersections = throttle(
      this._checkForIntersections.bind(this), this.THROTTLE_TIMEOUT);

  // Private properties.
  this._callback = callback;
  this._observationTargets = [];
  this._queuedEntries = [];
  this._rootMarginValues = this._parseRootMargin(options.rootMargin);

  // Public properties.
  this.thresholds = this._initThresholds(options.threshold);
  this.root = options.root || null;
  this.rootMargin = this._rootMarginValues.map(function(margin) {
    return margin.value + margin.unit;
  }).join(' ');
}
Creates the global IntersectionObserver constructor.
https://wicg.github.io/IntersectionObserver/#intersection-observer-interface
@param {Function} callback The function to be invoked after intersection
    changes have queued. The function is not invoked if the queue has
    been emptied by calling the `takeRecords` method.
@param {Object=} opt_options Optional configuration options.
@constructor