function isCircObj( obj, seen ) {
	var keys;
	var val;
	var i;

	seen.push( obj );
	keys = objectKeys( obj );
	if ( keys.length === 0 ) {
		return false;
	}
	for ( i = 0; i < keys.length; i++ ) {
		val = obj[ keys[ i ] ];
		if ( isObject( val ) && ( contains( seen, val ) || isCircObj( val, seen ) ) ) { // eslint-disable-line max-len
			return true;
		}
	}
	seen.pop( obj );
	return false;
}
Tests if an object contains a circular reference by recursively traversing object keys.

@private
@param {Object} obj - object to test
@param {Array} seen - array of seen objects
@returns {boolean} boolean indicating whether object contains a circular reference