Why is this filter function not working as expected?

This code:

let rId = requested.map((r) => r._id.toString());
console.log(rId);

let cId = confirmed.map((c) => c._id.toString());
console.log(cId);

rId.filter((r) => !cId.includes(r));
console.log(rId);

Prints this:

[
  '63bcc18d083dd2c66679e160',
  '63bcc331083dd2c66679e278',
  '63bdaef8f115ae565ac564c5'
]

[ '63bdaef8f115ae565ac564c5' ]

[
  '63bcc18d083dd2c66679e160',
  '63bcc331083dd2c66679e278',
  '63bdaef8f115ae565ac564c5'
]

Why is the last item in the array not filtered when the function returns a false statement?

What I’m expecting to be printed is this:

[
  '63bcc18d083dd2c66679e160',
  '63bcc331083dd2c66679e278',
  '63bdaef8f115ae565ac564c5'
]

[ '63bdaef8f115ae565ac564c5' ]

[
  '63bcc18d083dd2c66679e160',
  '63bcc331083dd2c66679e278', // <-- item removed
]

>Solution :

Filter does not mutate the array it is called on.

You need to set the result of the filter to a value;

rId = rId.filter((r) => !cId.includes(r));