discriminated union using non-literal-type

[kim-jaedeok]

🔎 Search Terms

"discriminated union", "discriminated union using non-literal-type"

🕗 Version & Regression Information

typescript@5.2.2

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAYg9nKBeKBvAUFKBDAXFAZ2ACcBLAOwHMBuTKAI3yLKtoF9bRIoAhbY5Gjp4o5AK4BbehGK0sjUZOmz0HdOi7QAqhTjlB8RAB9e-WugBmY8gGNgpPVGAQiAClQ4ANAyht8O8j0ASiEsUgsoV004COxkJBQAImYKSkSQjCwsbDks+ly2VXQgA

💻 Code

type Foo = {
  a: string;
  b: string;
};
type Bar = {
  a: number;
  b: number;
};

type Uinon = Foo | Bar;

function test({ a, b }: Uinon) {
  if (typeof a === "string") {
    a; // a is string
    b; // b is string | number
  }
}

🙁 Actual behavior

Inside the test function, b's type does not narrow down to string when a type is string.
why discriminated union only works with literal type?

🙂 Expected behavior

i think b's type should be string

Additional information about the issue

https://stackoverflow.com/questions/78707864/type-narrowing-doesnt-work-when-using-a-non-type-literal-field-as-a-tag/78707873#78707873

[jcalz]

Duplicate of #30506 and others linked within.

Aside: please correct "none-literal" to be "non-literal" so that anyone searching will find it.

[typescript-bot]

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