nl_tokens: On success check whether the target object is indeed non - extensible .
pl_tokens: function ( ) { var trap = this . getTrap ( "preventExtensions" ) ; if ( trap === undefined ) { // default forwarding behavior return Reflect . preventExtensions ( this . target ) ; } var success = trap . call ( this . <mask> , this . target ) ; success = ! ! success ; // coerce to Boolean if ( success ) { if ( Object_isExtensible ( this . target ) ) { throw new TypeError ( "can't report extensible object as non-extensible: " + this . target ) ; } } return success ; }