old_contents: /**
  * @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 getKeys = require( '@stdlib/utils/keys' );
  var copy = require( '@stdlib/utils/copy' );
  var isError = require( '@stdlib/assert/is-error' );
  var typeName = require( './type.js' );
  
  
  // MAIN //
  
  /**
  * Returns a JSON representation of an error object.
  *
  * @param {(Error|TypeError|SyntaxError|URIError|ReferenceError|RangeError|EvalError)} err - error to serialize
  * @throws {TypeError} first argument must be an error object
  * @returns {Object} JSON representation
  *
  * @example
  * var err = new Error( 'beep' );
  * var json = toJSON( err );
  * // returns <Object>
  */
  function toJSON( err ) {
  	var keys;
  	var out;
  	var i;
  	if ( !isError( err ) ) {
  		throw new TypeError( 'invalid argument. Must provide an error object. Value: `' + err + '`.' );
  	}
  	out = {};
  
  	// Guaranteed properties:
  	out.type = typeName( err );
  	out.message = err.message;
  
  	// Possible general error properties...
  	if ( err.name ) {
  		out.name = err.name;
  	}
  	if ( err.stack ) {
  		out.stack = err.stack;
  	}
  	// Possible Node.js (system error) properties...
  	if ( err.code ) {
  		out.code = err.code;
  	}
  	if ( err.errno ) {
  		out.errno = err.errno;
  	}
  	if ( err.syscall ) {
  		out.syscall = err.syscall;
  	}
  	// Any enumerable properties...
  	keys = getKeys( err );
  	for ( i = 0; i < keys.length; i++ ) {
  		out[ keys[i] ] = copy( err[ keys[i] ] );
  	}
  	return out;
  }
  
  
  // EXPORTS //
  
  module.exports = toJSON;
  
new_contents: /**
  * @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 getKeys = require( '@stdlib/utils/keys' );
  var copy = require( '@stdlib/utils/copy' );
  var isError = require( '@stdlib/assert/is-error' );
  var typeName = require( './type.js' );
  
  
  // MAIN //
  
  /**
  * Returns a JSON representation of an error object.
  *
  * @param {(Error|TypeError|SyntaxError|URIError|ReferenceError|RangeError|EvalError)} err - error to serialize
  * @throws {TypeError} first argument must be an error object
  * @returns {Object} JSON representation
  *
  * @example
  * var err = new Error( 'beep' );
  * var json = toJSON( err );
  * // returns {...}
  */
  function toJSON( err ) {
  	var keys;
  	var out;
  	var i;
  	if ( !isError( err ) ) {
  		throw new TypeError( 'invalid argument. Must provide an error object. Value: `' + err + '`.' );
  	}
  	out = {};
  
  	// Guaranteed properties:
  	out.type = typeName( err );
  	out.message = err.message;
  
  	// Possible general error properties...
  	if ( err.name ) {
  		out.name = err.name;
  	}
  	if ( err.stack ) {
  		out.stack = err.stack;
  	}
  	// Possible Node.js (system error) properties...
  	if ( err.code ) {
  		out.code = err.code;
  	}
  	if ( err.errno ) {
  		out.errno = err.errno;
  	}
  	if ( err.syscall ) {
  		out.syscall = err.syscall;
  	}
  	// Any enumerable properties...
  	keys = getKeys( err );
  	for ( i = 0; i < keys.length; i++ ) {
  		out[ keys[i] ] = copy( err[ keys[i] ] );
  	}
  	return out;
  }
  
  
  // EXPORTS //
  
  module.exports = toJSON;
  
subject: Update doctest