User-defined type guards with ternary operator

[k8w]

Search Terms

type guards ternary

Suggestion

Support to use dser-defined type guards with ternary operator

Use Cases

More complex type guards function

Examples

function checkAnimal(animal: Animal)
  : animal is Cat ? 'Cat'
    : animal is Dog ? 'Dog'
    : animal is Animal
{
  // ...
}

let value: any;
let checkResult = checkAnimal(animal);

if(checkResult === 'Cat'){
  // value is Cat now
}
else if(checkResult === 'Dog'){
  // value is Dog now
}
else if(checkResult){
  // value is other Animal
}
else{
  // value is not Animal
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[zpdDG4gta8XKpMCd]

conditional types can do it:

   type NameOfAnimal<A> = A extends Cat ? 'cat' : A extends Dog ? 'dog' : A extends Animal ? 'animal' : never;
   function checkAnimal<A extends Animal>(animal: A): NameOfAnimal<A> { ... }

[nmain]

conditional types can do it:

That won't work as a type guard, since TS will statically infer <A> (or you'll need to statically specify it).

http://www.typescriptlang.org/play/#src=interface%20Animal%20%7B%0D%0A%20%20%20%20%20%20%20%20ugly%3A%20boolean%3B%0D%0A%7D%0D%0Ainterface%20Dog%20extends%20Animal%20%7B%0D%0A%20%20%20%20%20%20%20%20noisy%3A%20boolean%3B%0D%0A%7D%0D%0Ainterface%20Cat%20extends%20Animal%20%7B%0D%0A%20%20%20%20%20%20%20%20annoying%3A%20boolean%3B%0D%0A%7D%0D%0A%0D%0A%0D%0Atype%20NameOfAnimal%3CA%3E%20%3D%20A%20extends%20Cat%20%3F%20'cat'%20%3A%20A%20extends%20Dog%20%3F%20'dog'%20%3A%20A%20extends%20Animal%20%3F%20'animal'%20%3A%20never%3B%0D%0Adeclare%20function%20checkAnimal%3CA%20extends%20Animal%3E(animal%3A%20A)%3A%20NameOfAnimal%3CA%3E%3B%0D%0A%0D%0Adeclare%20const%20a%3A%20Animal%0D%0Aif%20(checkAnimal(a)%20%3D%3D%3D%20%22dog%22)%20%7B%0D%0A%0D%0A%7D%0D%0A

[nmain]

Seems to be a duplicate of #30698

[zpdDG4gta8XKpMCd]

your example is not type narrowing it's type extending (into unknown) although i see what you mean, it looks like a poor design to me, sorry for being rude

problem in in using inheritance (from Animal to whatever) which is always an open ended question

if you defined it more strictly: type Animal = Cat | Dog it would not be a problem

which throws us to the expression problem which practically says that oop is a bad idea in general