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Type narrowing context is lost within closures #46118

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#56908
Closed
Type narrowing context is lost within closures#46118
#56908
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DuplicateAn existing issue was already created

Description

@ramyhhh
Rami Hanano (ramyhhh)
opened on Sep 28, 2021

Bug Report

🔎 Search Terms

typescript union typed narrowing lost in closures
type narrowing closures

🕗 Version & Regression Information

Typescript versions 4.4.3 and still exists on Nightly 4.5.0-dev

⏯ Playground Link

Playground link with relevant code

💻 Code

type MyUnion = { type: 'left', propertyExistsOnLeft: string } | { type: 'right', propertyExistsOnRight: string }

let x = {} as MyUnion;

switch (x.type) {
    case 'left':
        const y = x.propertyExistsOnLeft; //all good here ...
        console.log(() => x.propertyExistsOnLeft) //showing error "Property 'propertyExistsOnLeft' does not exist on type 'MyUnion'"
        break;
}

🙁 Actual behavior

Type narrowing is working as expected when following control flow, however when the variable is used inside a closures scope (arrow functions or normal functions) it reverts back to it's declared type losing all narrowing inferred from the containing scope.

🙂 Expected behavior

Type narrowing should continue to work inside the context of closures.

Activity

  1. Rami Hanano (ramyhhh)
    changed the title [-]Type narrowing lost within closures[/-] [+]Type narrowing context is lost within closures[/+] on Sep 28, 2021

  2. MartinJohns commented on Sep 28, 2021

    @MartinJohns
    Martin Johns (MartinJohns)
    on Sep 28, 2021
    Contributor

    And another duplicate of #9998.

  3. ramyhhh commented on Sep 28, 2021

    @ramyhhh
    Rami Hanano (ramyhhh)
    on Sep 28, 2021
    Author

    Yeah, that issue looks like a master issue collecting all control flow analysis issues :)

  4. MartinJohns commented on Sep 28, 2021

    @MartinJohns
    Martin Johns (MartinJohns)
    on Sep 28, 2021
    Contributor

    The exact same issue you describe comes up again and again and again. I probably have seen at least 50 issues already. It's all due to the difficulties described in #9998.

  5. fatcerberus commented on Sep 28, 2021

    @fatcerberus
    Bruce Pascoe (fatcerberus)
    on Sep 28, 2021

    Yeah, that issue looks like a master issue collecting all control flow analysis issues :)

    More specifically, control flow analysis issues across function boundaries. Which is not an easy problem to solve.

  6. Andrew Branch (andrewbranch)
    added
    DuplicateAn existing issue was already created
    on Oct 1, 2021
  7. TypeScript Bot (typescript-bot)
    closed this as completedon Oct 3, 2021

  8. typescript-bot commented on Oct 3, 2021

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Oct 3, 2021
    Contributor

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

  9. microsoft
    locked as resolved and limited conversation to collaborators on Oct 21, 2025
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