TS 5.4 has a feature called “Preserved Narrowing in Closures Following Last Assignments”, but it says,

Note that narrowing analysis doesn’t kick in if the variable is
assigned anywhere in a nested function. This is because there’s no way
to know for sure whether the function will be called later.

It gives the following code to explain that,

function printValueLater(value: string | undefined) {
    if (value === undefined) {
        value = "missing!";
    }
    setTimeout(() => {
        // Modifying 'value', even in a way that shouldn't affect
        // its type, will invalidate type refinements in closures.
        value = value;
    }, 500);
    setTimeout(() => {
        console.log(value.toUpperCase());
        //          ~~~~~
        // error! 'value' is possibly 'undefined'.
    }, 1000);
}

But I find that even if I don’t do the assignment, e.g. comment out that assignment call I still get “error! ‘value’ is possibly ‘undefined’.” Why is that ?

function printValueLater(value: string | undefined) {
    if (value === undefined) {
        value = "missing!";
    }
    setTimeout(() => {
        console.log(value.toUpperCase());
        //          ~~~~~
        // I still get 'value' is possibly 'undefined'.
    }, 1000);
}