Type narrowing not working with multiple adjacent case statements

[johan13]

🔎 Search Terms

"switch", "type narrowing"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about narrowing and switch statements

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABMOcAUBDATgcwFyIDOUWMYOiAPouACYCmwZ9tAlIgN4BQiviMwRGgCE2HOygALLHADuiMPXkBRLDKxpWAbi48+hWTCgRJQkiHrtufG4ggZC9RGIB0UOAFUADl-pYAwg70mogAvOGIAEQAgpEEAPTxiADyANJ6trZY9FAgWEgxkToZtvaOzrhunj5+gY4h4aFRAEJxiImIfuqIACoAygCMABwADAAsQwQA5GJT-ISIXnCEhDAARgA2AJ6IU3SMzLRTLiU2ZU6u7t6+AUENEZH+bR1dcFi9g6MT07Pzi8urTY7PZgBhMRRHE6ZXjnCo4KrXWp3diNKIAEWeSTSp2h2Vy+RaABpEP5EG9EBidDYAL66Gx4vIFABycCgiGixOaxNJ5MpXFpQA

💻 Code

function foo(arg: string | undefined) {
    if (!arg) throw new Error();

    switch (true) {
        case arg.toUpperCase() === "A": // OK
            return "A";

        case arg.toUpperCase() === "B": // error TS18048: 'arg' is possibly 'undefined'.
        case arg.toUpperCase() === "C": // error TS18048: 'arg' is possibly 'undefined'.
        case arg.toUpperCase() === "D": // OK
            return "B, C or D";
    }

    return "Not A, B, C or D";
}

🙁 Actual behavior

In two of the four case statements, the arg variable has the type string | undefined instead of string.

🙂 Expected behavior

I would expect the arg variable to have the same type in all four case statements.

Additional information about the issue

No response

[RyanCavanaugh]

Not fixed in Nightly

cc @Andarist

[Andarist]

I'll work on this, thanks for the ping.

[Andarist]

Interestingly... it's not a new issue. It's just a new manifestation of it: TS playground. I think that all of those 3 case clauses should report the same type for arg. Of course, this particular code doesn't make a lot of sense 😅 but based on the initial investigation I believe it shows the same root cause. Those fallthrough cases are assigned an unreachable flow and that's why the flow type for arg can't be narrowed down using the pre-switch case flow (they are not connected). I'll continue investigating this.

[johan13]

You are right - this is not new. I verified this behaviour all the way back to Typescript 3.0 before I opened the issue.

I just found it now because Typescript 5.3 adds better support for the switch (true) pattern and I wanted to play around with it.

One interesting thing I found is that the first group of case statements in a switch works as expected, but in subsequent groups only the last case gets the narrowed type.

[Andarist]

A more sane repro of this problem that doesn't rely on switch (true): TS playground

[jakebailey]

That's unrelated; that's just #9998.

[fatcerberus]

Wait, how is that 9998? There’s no function call or closure there, bar is narrowed and then later the narrowing disappears in the same scope.

[jakebailey]

I responded to another user who deleted their post. I have no doubt that @Andarist knows all about #9998 😄

[jakebailey]

From my email...

[johan13]

I just stumbled across a similar issue. Playground

function foo(arg: string | undefined) {
    switch (true) {
        case arg === undefined:
            throw new Error();

        case arg.startsWith("x"): // error TS18048: 'arg' is possibly 'undefined'.
            return "A";

        default:
            return "B";
    }
}

Is it intended behavior that that I get an error here?