String union types narrowed to falsy should narrow string to ""

[JoshuaKGoldberg]

Bug Report

🔎 Search Terms

string truthy falsy narrow literal empty

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about narrowing
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

let value = Math.random() > 0.5 && "Some Name";

if (!value) {
  value; // Type: false | string, but should be false | ""
}

🙁 Actual behavior

Inside the narrowed block, value should be false | "", because no other string value is falsy.

🙂 Expected behavior

It's the more general false | string instead.

This was a hard issue to search for; #41503, #33878, and #31156 seemed related but not duplicate to me.

Shoutout to RyanCavanaugh for sending me a code snippet that looked like this -- no good deed goes unpunished in the issue tracker 😄

[RyanCavanaugh]

This seems like a good idea. I'm wondering what it'd break; would love a draft PR so we could run it over real-world code to see.

[JoshuaKGoldberg]

@RyanCavanaugh following up since typescript-bot warned me this issue isn't accepted: does the non-negative results from the community code test suite and perf tests in the PR mean this looks like something you'd take in?

[ahejlsberg]

We've considered this one before, but we never implemented it because we can only narrow in the falsy case (narrowing in the truthy case would require negated types such as number & not 0). Other than the abstract example, are there real world scenarios this would enable?

[JoshuaKGoldberg]

Shoot sorry for missing this @ahejlsberg, it dropped off my radar. I've seen the occasional snippet of code that uses "" instead of null or undefined in unions of response types.

type Result = "ACK" | "ERR" | "";

declare const result: Result;

if (result) {
  // handle that a result was received
} else {
  // couldn't get a result
}

I don't have any examples that are open source, though.