text: /**
  * @license Apache-2.0
  *
  * Copyright (c) 2018 The Stdlib Authors.
  *
  * 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.
  */
  
  'use strict';
  
  // MODULES //
  
  var isObject = require( '@stdlib/assert/is-plain-object' );
  var hasOwnProp = require( '@stdlib/assert/has-own-property' );
  var indexOf = require( '@stdlib/utils/index-of' );
  
  
  // VARIABLES //
  
  var returns = [ 'values', 'keys', '*' ];
  
  
  // MAIN //
  
  /**
  * Validates function options.
  *
  * @private
  * @param {Object} opts - destination object
  * @param {Options} options - function options
  * @param {*} [options.thisArg] - execution context
  * @param {string} [options.returns] - output format
  * @returns {(Error|null)} null or an error object
  *
  * @example
  * var opts = {};
  * var options = {
  *     'returns': '*',
  *     'thisArg': {}
  * };
  * var err = validate( opts, options );
  * if ( err ) {
  *     throw err;
  * }
  */
  function validate( opts, options ) {
  	if ( !isObject( options ) ) {
  		return new TypeError( 'invalid argument. Options must be an object. Value: `' + options + '`.' );
  	}
  	if ( hasOwnProp( options, 'thisArg' ) ) {
  		opts.thisArg = options.thisArg;
  	}
  	if ( hasOwnProp( options, 'returns' ) ) {
  		opts.returns = options.returns;
  		if ( indexOf( returns, opts.returns ) === -1 ) {
  			return new TypeError( 'invalid option. `returns` option must be one of the following: "' + returns.join( '", "' ) + '". Option: `' + opts.returns + '`.' );
  		}
  	}
  	return null;
  }
  
  
  // EXPORTS //
  
  module.exports = validate;
  