code: function mergeOptions (
    parent,
    child,
    vm
  ) {
    {
      checkComponents(child);
    }
  
    if (typeof child === 'function') {
      child = child.options;
    }
  
    normalizeProps(child, vm);
    normalizeInject(child, vm);
    normalizeDirectives(child);
  
    // Apply extends and mixins on the child options,
    // but only if it is a raw options object that isn't
    // the result of another mergeOptions call.
    // Only merged options has the _base property.
    if (!child._base) {
      if (child.extends) {
        parent = mergeOptions(parent, child.extends, vm);
      }
      if (child.mixins) {
        for (let i = 0, l = child.mixins.length; i < l; i++) {
          parent = mergeOptions(parent, child.mixins[i], vm);
        }
      }
    }
  
    const options = {};
    let key;
    for (key in parent) {
      mergeField(key);
    }
    for (key in child) {
      if (!hasOwn(parent, key)) {
        mergeField(key);
      }
    }
    function mergeField (key) {
      const strat = strats[key] || defaultStrat;
      options[key] = strat(parent[key], child[key], vm, key);
    }
    return options
  }
docs: Merge two option objects into a new one.
  Core utility used in both instantiation and inheritance.