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enable type guards over string literal types #6028

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enable type guards over string literal types#6028
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CommittedThe team has roadmapped this issueDomain: Literal TypesUnit types including string literal types, numeric literal types, Boolean literals, null, undefinedFixedA PR has been merged for this issueSuggestionAn idea for TypeScript
Milestone
TypeScript 2.0

Description

@zpdDG4gta8XKpMCd
ZpdDG4gta (zpdDG4gta8XKpMCd)
opened on Dec 10, 2015

Now that the string literal types #5185 are merged in, the type guards based on them would be an awesome addition:

by Anders Hejlsberg (@ahejlsberg):

Type guards could relate the common kind property to the corresponding object type in the union and narrow the type appropriately in the guarded block.

The original issue #1003 got closed with the following comment:

the remaining part is extending the type guards to support the string literal types. but this should be tracked by a different issue.

A quick search suggests that there isn't a ticket for this yet. So here we go.

Activity

  1. Mohamed Hegazy (mhegazy)
    added
    SuggestionAn idea for TypeScript
    CommittedThe team has roadmapped this issue
    on Dec 10, 2015
  2. Mohamed Hegazy (mhegazy)
    assigned
    Daniel Rosenwasser (DanielRosenwasser)
    on Dec 10, 2015

  3. DanielRosenwasser commented on Dec 10, 2015

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Dec 10, 2015
    Member

    Thanks for filing Aleksey-Bykov!

  4. Daniel Rosenwasser (DanielRosenwasser)
    added this to the TypeScript 1.8 milestone on Dec 10, 2015
  5. Daniel Rosenwasser (DanielRosenwasser)
    added
    Domain: Literal TypesUnit types including string literal types, numeric literal types, Boolean literals, null, undefined
    on Jan 4, 2016
  6. Mohamed Hegazy (mhegazy)
    removed this from the TypeScript 1.8 milestone on Jan 7, 2016

  7. Eyas commented on Jan 22, 2016

    @Eyas
    Eyas (Eyas)
    on Jan 22, 2016
    Contributor

    Is this no longer intended for TypeScript 1.8?

  8. DanielRosenwasser commented on Jan 22, 2016

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Jan 22, 2016
    Member

    I predict that'll be going in for 2.0. We're still working on the core functionality of string literal types, and how we want things to flow throughout the type system. If we do that right, this will be much easier and might enable much more interesting features down the road.

  9. Eyas commented on Jan 22, 2016

    @Eyas
    Eyas (Eyas)
    on Jan 22, 2016
    Contributor

    Daniel Rosenwasser (@DanielRosenwasser) is there an issue/set of issues to track about the type system work?

  10. zpdDG4gta8XKpMCd commented on Jan 22, 2016

    @zpdDG4gta8XKpMCd
    ZpdDG4gta (zpdDG4gta8XKpMCd)
    on Jan 22, 2016
    Author

    what are the more exciting features?

  11. DanielRosenwasser commented on Jan 22, 2016

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Jan 22, 2016
    Member

    Eyas (@Eyas) Yes. 😉

  12. DanielRosenwasser commented on Jan 22, 2016

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Jan 22, 2016
    Member

    Aleksey-Bykov basically the idea that you'd be able to not only narrow from strings to string literals, but you'd be able to use them as tags, as discussed in #1003.

  13. zpdDG4gta8XKpMCd commented on Jan 22, 2016

    @zpdDG4gta8XKpMCd
    ZpdDG4gta (zpdDG4gta8XKpMCd)
    on Jan 22, 2016
    Author

    good, that is what this issue is about, I thought there might be more to that

  14. shaharmor commented on Mar 29, 2016

    @shaharmor
    Shahar Mor (shaharmor)
    on Mar 29, 2016

    Not sure if its listed anywhere, but i think this should show an error: (Hopefully in the IDE)

    // declare
type options = 'option1' | 'option2';

interface A {
   possibleOptions: options;
}

/// use (given class B that implements interface A)
var x = new B();
if (x.possibleOptions === 'option3') {
  // there is no way of getting here is all possible values are either 'option1' or 'option2', should error
}
  15. Mohamed Hegazy (mhegazy)
    unassigned
    Daniel Rosenwasser (DanielRosenwasser)
    on May 4, 2016

  16. DanielRosenwasser commented on Jun 23, 2016

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Jun 23, 2016
    Member

    Can we say that this has been fixed by #9163?

  17. zpdDG4gta8XKpMCd commented on Jun 23, 2016

    @zpdDG4gta8XKpMCd
    ZpdDG4gta (zpdDG4gta8XKpMCd)
    on Jun 23, 2016
    Author

    yes, thanks
    On Jun 23, 2016 02:16, "Daniel Rosenwasser" notifications@github.com
    wrote:

    Can we say that this has been fixed by #9163
    #9163?

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  18. Mohamed Hegazy (mhegazy)
    added this to the TypeScript 2.0 milestone on Jun 23, 2016
  19. Mohamed Hegazy (mhegazy)
    added
    FixedA PR has been merged for this issue
    on Jun 23, 2016
  20. microsoft
    locked and limited conversation to collaborators on Jun 19, 2018
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