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Infer Type Guard Return Values #5101

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Infer Type Guard Return Values#5101
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Domain: lib.d.tsThe issue relates to the different libraries shipped with TypeScriptHelp WantedYou can do thisSuggestionAn idea for TypeScript
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Community (deprecated)

Description

@Gaelan
Gaelan Steele (Gaelan)
opened on Oct 4, 2015

If a function returns a type guard, either the result of an instanceofor the return value of another type guard function, it could be inferred as being a type guard itself. However, it is simply inferred as a boolean.

This change would allow the following code to work:

// The interface below should probably be merged into lib.d.ts and friends.
interface Array<T> {
    filter<O extends T>(callbackfn: (value: T, index: number, array: T[]) => value is O, thisArg?: any): O[];
}

declare class Foo { bar: number }

declare var blah: Object[];

blah.filter(object => object instanceof Foo)[0].bar // Property bar does not exist on type object

Activity

  1. Ryan Cavanaugh (RyanCavanaugh)
    added
    SuggestionAn idea for TypeScript
    In DiscussionNot yet reached consensus
    on Oct 5, 2015

  2. mhegazy commented on Oct 5, 2015

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on Oct 5, 2015
    Contributor

    We should add the overload to the library regardless of this suggestion. Gaelan Steele (@Gaelan) feel free to send us PR for updating the library.

  3. Ryan Cavanaugh (RyanCavanaugh)
    added
    Help WantedYou can do this
    Domain: lib.d.tsThe issue relates to the different libraries shipped with TypeScript
    and removed
    In DiscussionNot yet reached consensus
    on Oct 5, 2015
  4. Ryan Cavanaugh (RyanCavanaugh)
    added this to the milestone on Oct 5, 2015

  5. RyanCavanaugh commented on Oct 5, 2015

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Oct 5, 2015
    Member

    Accepting PRs for the lib.d.ts overload.

    The more complex portion of inferring type guards based on return statements was declined based on complexity. It's fun and easy to write a curried version of this that works today (with the additional overload for filter):

    declare class Foo { bar: number }

declare var blah: Object[];

function isInstance<T>(ctor: new(...args: any[]) => T): (x: any) => x is T {
    return <(x: any) => x is T>(x => x instanceof ctor);
} 
blah.filter(isInstance(Foo))[0].bar // Property bar does exist :)

  6. zpdDG4gta8XKpMCd commented on Apr 3, 2017

    @zpdDG4gta8XKpMCd
    ZpdDG4gta (zpdDG4gta8XKpMCd)
    on Apr 3, 2017

    can't quite see how changing libs.d.ts is able to fix this problem, mind sharing idea?

  7. mhegazy commented on Apr 5, 2017

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on Apr 5, 2017
    Contributor

    This is not a general fix, this is only a fix for array.filter. By changing the declaration you get the contextual type for the the function expression at the call.

  8. mhegazy commented on May 2, 2018

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on May 2, 2018
    Contributor

    The overload for Array.filter should be in already. closing.

  9. Mohamed Hegazy (mhegazy)
    closed this as completedon May 2, 2018

  10. mhegazy commented on May 2, 2018

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on May 2, 2018
    Contributor

    #16069 now tracks inferring the type guard automatically.

  11. microsoft
    locked and limited conversation to collaborators on Jul 31, 2018
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