Type Narrowing Incorrect on Exported Variable

[ChrisMeyer7088]

🔎 Search Terms

Narrowing Export

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about 5.0 and higher releases

⏯ Playground Link

https://www.typescriptlang.org/play?#code/DYUwLgBARsCGAWAuCBnMAnAlgOwOYQB8IBXbAExADMcQyBuAKAc0ogAoBCGBASgjHjoA9gHcI2EGICi6YejYByACrxMKCCFlD0EMkJDrsQyNlhaxYAJ4AHEAp5MQAD2vbIAbwYRoceBAD0-hAASuDE6NjqVraoGDj4RKQU1BJkADSo8ELEwGTQIOJmwiK0-EKxWHgMAL5AA

💻 Code

let blah: string | undefined;

if (!blah) throw new Error('This error does not narrow type')

export {
  blah // Returns type string | undefined, should be narrowed to string
}

🙁 Actual behavior

Expected the throwing of an error to type narrow the exported variable, but it instead held onto its initial typing.

🙂 Expected behavior

Throwing an error should type narrow exported variables

Additional information about the issue

No response

[RyanCavanaugh]

This is the correct behavior. It's possible to observe blah before the if, e.g.:

// self-import in foo.mjs
import { blah as self_blah } from "./foo.mjs";
let blah;
// prints "undefined"
console.log(self_blah);
if (!blah) throw new Error('This error does not narrow type')
export {
  blah
}

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.