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Type predicate assertion does not carry over when used in another function #51326

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Type predicate assertion does not carry over when used in another function#51326
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Description

@yogiduzit
Yogesh Verma (yogiduzit)
opened on Oct 27, 2022

Bug Report

🔎 Search Terms

  • predicate
  • type predicate
  • type predicate return

🕗 Version & Regression Information

  • This is NOT a crash
  • This changed between versions ______ and _______
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
  • I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Animal {
  name: string;
}

interface Bird extends Animal {
  wingSpan: string;
}

/**
 * Asserts that an animal is of type T if condition is true for that animal.
 */
function isAnimalOfType<T extends Animal>(animal: Animal, condition: (animal: T) => boolean): animal is T {
  return condition(animal as T);
}

function isBird(animal: Animal) {
  return isAnimalOfType<Bird>(animal, (bird) => Boolean(bird.wingSpan));
}

function getWingspan(bird: Bird) {
  return bird.wingSpan;
}

const animals = [{ name: "a" }, { name: "a", wings: "3ft" }] as Animal[];

function checkAnimals(animals: Animal[]) {
  return animals.map(animal => {
    console.log(animal);
    if (isBird(animal)) {
      console.log(getWingspan(animal));
    }
  })
}

🙁 Actual behavior

The call to getWingspan shows an error saying Argument of type 'Animal' is not assignable to parameter of type 'Bird'.

🙂 Expected behavior

The function isAnimalOfType already asserts that the Animal will be of type T. So, why does isBird not automatically assert that?

Activity

  1. fatcerberus commented on Oct 27, 2022

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Oct 27, 2022

    Type predicates are not automatically inferred. You need to explicitly declare isBird as returning animal is Bird.

  2. jcalz commented on Oct 27, 2022

    @jcalz
    Joe Calzaretta (jcalz)
    on Oct 27, 2022
    Contributor

    Duplicate of #10734

  3. Ryan Cavanaugh (RyanCavanaugh)
    added
    DuplicateAn existing issue was already created
    on Oct 27, 2022
  4. TypeScript Bot (typescript-bot)
    closed this as completedon Oct 29, 2022

  5. typescript-bot commented on Oct 29, 2022

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Oct 29, 2022
    Contributor

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

  6. danvk commented on May 15, 2024

    @danvk
    Dan Vanderkam (danvk)
    on May 15, 2024
    Contributor

    This was marked as a duplicate of #10734, which was closed by #57465. But that PR only let "identifier" type predicates flow, not assertion predicates. The example in this issue is still broken.

    This specific issue would be fixed by #58495, see playground.

  7. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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