abstract class Animal
{
    get isDog(): this is Dog;
}

class Dog extends Animal
{
    get isDog(): this is Dog { return true; }
    bark() { console.log("woof"); }
}

let animal = new Dog();
if (animal.isDog)
{
   animal.bark()
}

This use-case was suggested by Ryan Cavanaugh (@RyanCavanaugh) when he opened #1007 but seems to be unsupported as of TypeScript 1.6.

Activity

Anders Hejlsberg (ahejlsberg)
added
SuggestionAn idea for TypeScriptAn idea for TypeScript
on Nov 24, 2015
ahejlsberg commented on Nov 24, 2015
Anders Hejlsberg (ahejlsberg)
on Nov 24, 2015
Member
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Looks like a good suggestion to me.
Reacted by Yuta Kawabe
Ryan Cavanaugh (RyanCavanaugh)
added
In DiscussionNot yet reached consensusNot yet reached consensus
on Nov 24, 2015
Wesley Wigham (weswigham)
closed this as completedin #5906 on Dec 10, 2015
Daniel Rosenwasser (DanielRosenwasser)
added
FixedA PR has been merged for this issueA PR has been merged for this issue
on Dec 10, 2015
Mohamed Hegazy (mhegazy)
added this to the TypeScript 1.8 milestone on Dec 10, 2015
Mohamed Hegazy (mhegazy)
removed
In DiscussionNot yet reached consensusNot yet reached consensus
on Dec 10, 2015
robyoder commented on Mar 24, 2016
Rob Yoder (robyoder)
on Mar 24, 2016
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It looks like this wasn't implemented as described. I can't seem to set a type guard on a getter like in the example above, only on regular functions. On a getter, I'm told "A type predicate is only allowed in return type position for functions and methods."
mhegazy commented on Mar 24, 2016
Mohamed Hegazy (mhegazy)
on Mar 24, 2016
Contributor
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that is correct. you can find more documentation in https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#this-based-type-guards
robyoder commented on Mar 24, 2016
Rob Yoder (robyoder)
on Mar 24, 2016
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I read that already, doesn't talk about this at all...
mhegazy commented on Mar 24, 2016
Mohamed Hegazy (mhegazy)
on Mar 24, 2016
Contributor
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User define guards are not supported in property declaration or acccessor (getter or setter). that as specified in the original proposal, but was cut from the implementation.
microsoft
locked and limited conversation to collaborators on Jun 19, 2018