/**
 * Copyright 2013-2014 Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * @providesModule onlyChild
 */
"use strict";

var ReactDescriptor = require("./ReactDescriptor");

var invariant = require("./invariant");

/**
 * Returns the first child in a collection of children and verifies that there
 * is only one child in the collection. The current implementation of this
 * function assumes that a single child gets passed without a wrapper, but the
 * purpose of this helper function is to abstract away the particular structure
 * of children.
 *
 * @param {?object} children Child collection structure.
 * @return {ReactComponent} The first and only `ReactComponent` contained in the
 * structure.
 */
function onlyChild(children) {
  ("production" !== process.env.NODE_ENV ? invariant(
    ReactDescriptor.isValidDescriptor(children),
    'onlyChild must be passed a children with exactly one child.'
  ) : invariant(ReactDescriptor.isValidDescriptor(children)));
  return children;
}

module.exports = onlyChild;
