/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule onlyChild
 */
'use strict';

var { isValidElement } = require('React');

var invariant = require('invariant');

/**
 * Returns the first child in a collection of children and verifies that there
 * is only one child in the collection.
 *
 * See https://facebook.github.io/react/docs/react-api.html#react.children.only
 *
 * 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 {ReactElement} The first and only `ReactElement` contained in the
 * structure.
 */
function onlyChild(children) {
  invariant(
    isValidElement(children),
    'React.Children.only expected to receive a single React element child.'
  );
  return children;
}

module.exports = onlyChild;