Getting Object is possibly 'null' even after null checking

Question

export async function getSingleUserQA(command: Command, period?: string[]) {
  let slackUserID;

  if (command && command.text) {
    slackUserID = command.text.match("/(?<=@)(.*?)(?=|)/g")[0];
  }
}

I am being flagged that 'command' is possibly 'null' or 'undefined'. How come if I am making sure that command exists and command.text exists?

`command.text` could be changed outside the method. https://stackoverflow.com/questions/518000/is-javascript-a-pass-by-reference-or-pass-by-value-language — Blundell, Feb 13 '22 at 20:53
@Blundell and yet would not matter because of the `if` check. — VLAZ, Feb 13 '22 at 20:54
It's multithreaded so it does matter. (`async`) The value can change between the check and the next line invocation. ... *I think* :D — Blundell, Feb 13 '22 at 20:58
@Blundell JS execution is *not* multithreaded in the vast majority of times. It could be *asynchronous* but still not parallel, since you have to explicitly opt into parallel execution and I sincerely doubt OP has done it. — VLAZ, Feb 13 '22 at 21:09

score 1 · Accepted Answer · answered Feb 13 '22 at 20:54

Look closely at the warning. It's not saying that command might not exist, but that this whole expression:

command.text.match("/(?<=@)(.*?)(?=|)/g")

might not.

What do you want to do if the regular expression has no match? Figure that out, and guard it appropriately.

if (command && command.text) {
    const match = command.text.match("/(?<=@)(.*?)(?=|)/g");
    if (match) {
        slackUserID = match[0];
    } else {
        // handle badly formatted command
    }
}

You should also probably do

let slackUserID: string;

or only use the result inside the conditional.

Getting Object is possibly 'null' even after null checking

1 Answers1