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Adding boolean type assertion changes type narrowing  #60190

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Adding boolean type assertion changes type narrowing #60190
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Not a DefectThis behavior is one of several equally-correct options

Description

@jauwesome
Jon Austin (jauwesome)
opened on Oct 10, 2024

🔎 Search Terms

Type Narrowing
Boolean
Undefined

🕗 Version & Regression Information

Version: 5.6.3
Regression: Unknown

⏯ Playground Link

https://www.typescriptlang.org/play/?noUnusedLocals=true&noUnusedParameters=true#code/GYVwdgxgLglg9mABFApgZygeTCgFMOOALkQwCcYwBzRAH0XABMVhKVGBKRAbwChEBiCAgzIAFpRoBeRAEJZBOAG5+gmMFxQJ1Ln0H7EZFFBBkkigHQQxAQzIBBKLgAMHFfoC+qgUZNnEAORMLGyMAbxevKCQsAjI6FAAKgDucPiEJOSSdAxgzKw4nDzeQiJQ4pIkAEaEADYoNkgy8oruahpakrol+r6m5oRWtg5Orm0CXr3G-YHBBezhHkA

💻 Code

function testOne(foo: string | undefined) {
    const thing = !!foo;
    if(thing) {
        // Foo type is String
        return foo.charAt(0);
    }
    return 'undefined'
}

function testTwo(foo: string | undefined) {
    const thing: boolean = !!foo;
    if(thing) {
         // Foo type is string | undefined
        return foo.charAt(0);
    }
    return 'undefined'
}

🙁 Actual behavior

foo is typed as string | undefined if thing has explicit type of boolean

🙂 Expected behavior

foo should be typed as string if thing has type of boolean just like if thing's type is inferred

Additional information about the issue

No response

Activity

  1. Jon Austin (jauwesome)
    changed the title [-]Adding boolean type assertion changes type narrowing for undefined[/-] [+]Adding boolean type assertion changes type narrowing [/+] on Oct 10, 2024
  2. Ryan Cavanaugh (RyanCavanaugh)
    added
    Not a DefectThis behavior is one of several equally-correct options
    on Oct 10, 2024

  3. RyanCavanaugh commented on Oct 10, 2024

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Oct 10, 2024
    Member

    This was an intentional boundary in #44730

  4. jauwesome commented on Oct 10, 2024

    @jauwesome
    Jon Austin (jauwesome)
    on Oct 10, 2024
    Author

    Ryan Cavanaugh (@RyanCavanaugh) Thanks for the quick response! Is there a specific reason why this choice was made? I don't see that listed. Regardless we can close the ticket.

  5. Jon Austin (jauwesome)
    closed this as completedon Oct 10, 2024

  6. RyanCavanaugh commented on Oct 10, 2024

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Oct 10, 2024
    Member

    IIRC it's possible to get into a type resolution circularity with this inference in a way that wouldn't really be solvable, so you need some "out" in cases where you're not using it for narrowing purposes

  7. jauwesome commented on Oct 11, 2024

    @jauwesome
    Jon Austin (jauwesome)
    on Oct 11, 2024
    Author

    Good to know! That makes sense. Thanks again for your help!

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