nodejs process.setgid, process.setuid behavior with fs module

Question

directory:

drwxrwxr-x  2 alex alex 4096 Aug  3 12:03 ./
drwxr-xr-x 17 alex alex 4096 Aug  3 11:18 ../
-rwx------  1 root root   19 Aug  3 11:24 privilegedStuff*
-rwxrwx---  1 root root   28 Aug  3 12:10 privilegedStuff1*
-rwxrwxr-x  1 alex alex  830 Aug  3 12:12 test.js*

test.js:

#!/usr/bin/env node
var fs = require('fs');
console.log('           user id: ', process.getuid());
console.log('          group id: ', process.getgid());
console.log(' user effective id: ', process.getegid());
console.log('group effective id: ', process.getegid());
console.log('\n switching user and group...\n');
process.setgid(1000);
process.setegid(1000);
process.setuid(1000);
process.seteuid(1000);
console.log('           user id: ', process.getuid());
console.log('          group id: ', process.getgid());
console.log(' user effective id: ', process.getegid());
console.log('group effective id: ', process.getegid());
console.log('\n output: \n');

console.log(fs.readFileSync('./privilegedStuff1', 'utf8'))

// this throws error as expected so I commented that
// console.log(fs.readFileSync('./privilegedStuff', 'utf8'))

privilegedStuff1:

content of privilegedStuff1

result:

alex@hp:/apps/test$ sudo ./test.js 
           user id:  0
          group id:  0
 user effective id:  0
group effective id:  0

 switching user and group...

           user id:  1000
          group id:  1000
 user effective id:  1000
group effective id:  1000

 output: 

content of privilegedStuff1

so what I don't understand is why node doesn't throw an error as it does nicely with privilegedStuff file? What am I missing?

alex@hp:/apps/test$ groups
alex adm cdrom sudo dip plugdev lpadmin sambashare
alex@hp:/apps/test$ cat privilegedStuff1
cat: privilegedStuff1: Permission denied
alex@hp:/apps/test$ sudo -s
root@hp:/apps/test# groups
root

Answer 1

In my test, I don't have such problem.

Can you enter the following command and show the result:

ls -l privilegedStuff1
id