Function mergeAssets

static/vuejs/vue.runtime.js:1070–1075

* Assets * * When a vm is present (instance creation), we need to do * a three-way merge between constructor options, instance * options and parent options.

(parentVal, childVal)

 * options and parent options.
 */
function mergeAssets (parentVal, childVal) {
  var res = Object.create(parentVal || null);
  return childVal
    ? extend(res, childVal)
    : res
}

config._assetTypes.forEach(function (type) {
  strats[type + 's'] = mergeAssets;