Typescript: Suggestion: Type guard generic types

[dsherret]

Would it be good to have type guarding on generic types? Or was this decided against?

Example 1

class Parent {
    prop1: string;
}

class Child extends Parent {
    childProp: string;
}

var t: Parent = new Child();

if (t instanceof Child) {
    t.childProp; // no error
}

function myFunction<T extends Parent>(item: T) {
    if (item instanceof Child) {
        item.childProp; // error
    }
}

I realize this example could be rewritten without generics. It's just an example :)

Example 2

Here's a scenario I came across when creating a wrapper class:

interface NedbItem {
    _id: string;
}

function isNedbItem(item: any): item is NedbItem {
    return item != null && typeof item._id === "string";
}

class DatabaseWrapper<T> {
    getItems() {
        const itemsFromDatabase: T[] = .....;

        itemsFromDatabase.forEach(i => this.stripNedbID(i));

        return itemsFromDatabase;
    }

    private stripNedbID(item: T) {
        if (isNedbItem(item))
        {
            delete item._id; // error, _id does not exist on T
        }
    }
}

Maybe a generic type with no constraint should be treated like an any type in this case? (Once type guards are fixed for any types -- See #4432)

This suggestion is probably very low on the priority scale.

I couldn't find this issue discussed elsewhere so sorry if it was.

[dsherret]

Just thinking about this a bit more. I guess this is not allowed for the same reason that this is not allowed:

function myFunction<T>(item: T) {
    const s = item as string; // error, neither T nor string is assignable to the other
}

I guess it does make sense to not allow this the more I think about it and it won't be needed in most scenarios.

I'm closing this issue.

[ziahamza]

There are still valid cases where this can be helpful, especially when generic type extends from a union type. E.g

class Cat {    }
class Dog {  }
class Node<T extends Cat | Dog> {
     data: T = null
     print() {
          if (this.data instanceof Dog) {
                // this.data now has the type Dog inside the block
          }
     }
} 

But the example doesnt seem to work today as typescript does not specialise the generic type even if it is a subset of the type it extends from

[mhegazy]

there is another problem in the use of this.data; type guards do not work on property look ups only variables. that is tracked by #1260

[RyanCavanaugh]

Accepting PRs.

The root cause here comes from two factors.

First is that given a (constrained) generic type parameter T extends U, we don't consider T to be assignable from a subtype of U. That rule is correct - it would be unsound to let you write code like this:

function clone<T extends Animal>(x: T) {
  return new Dog(); // Unsafe: Dog is subtype of Animal, but is not a subtype of T
}
clone(new Cat()); // Example invocation of other subtype

Second is that a type guard on an expression of type W to type V only succeeds if V is assignable to W. That rule is also correct -- typeguarding guarding a Cat with isDog should not succeed.

The rules do not combine favorably in the case of generics, though. We believe the correct fix is to take the apparent type of the generic type parameter when performing the assignability check in the second rule.

[JsonFreeman]

Thinking about @RyanCavanaugh's two root causes, why should type guarding Cat with isDog be illegal? What if the Cat is actually Cat & Dog at runtime?

[hesselink]

I have another use case for this. Simplified:

interface A { type : string }
interface B extends A { f () : B }
function isB (x : A) : x is B {
    return x.type === "B";
} 

// Doesn't work.
function g <T extends A> (x : T) : T {
    if (isB(x))
      return x.f()
}

// Works, but lets you return an `A` if passed a `B`.
function h (x : A) : A {
    if (isB(x))
      return x.f();

The function g was written with generics to enforce that you get back the type you passed in. So you can pass in either an A or a B, but when you pass a B, you will get back a B as well. However, after type isB check, typescript doesn't refine T to be a B, so the call to f fails to type check.

[tiagoefmoraes]

Another use case:

function foo<T extends string | number>(value: T) {
    return typeof value === "number" && isNaN(value);
    // on isNaN value still is "value: T extends string | number"
}

[masaeedu]

It seems like it already sort-of works if I explicitly write a generic function that returns the type guard:

class Cat { meow() { console.log("meow") } }
class Dog { woof() { console.log("woof") } }

class N<T extends Cat | Dog> {
    data: T = null
    print() {
        const data = this.data

        // This is fine
        if (is(data, Dog)) {
            return data.woof()
        }

        // Unfortunately, this is not. Looks
        // like narrowing isn't working.
        // return data.meow()

        // However, you can work around the
        // problem with this:
        if (is(data, Cat)) {
            return data.meow()
        }
    }
}

function is<T, TClass>(x: T, c: new () => TClass): x is T & TClass {
    return x instanceof c
}

Unfortunately, it looks like narrowing of the union doesn't work. If I add the explicit annotation const data: Cat | Dog the error goes away, so it looks like an unfortunate interaction between union narrowing and generics. In the meantime, we can just explicitly write if-clauses for each case instead of relying on narrowing.

[thorn0]

Why is this issue still open? All the examples here compile fine already.

[thorn0]

What about more advanced scenarios? Like this:

function foo<T>(a: T) {
  const b = a;
  if (typeof a === 'string') {
    console.log(b.toLowerCase()); // error
  }
}

It doesn't compile, however wouldn't it be safe to assume that T is string inside the if? Is there probably another issue where it's been discussed?

[rolfvandekrol]

I found another example of this problem where this goes wrong.

type A = { kind: "a" }
type B = { kind: "b" }

const a = (a: A): void => undefined
const b = (b: B): void => undefined

const c = <C extends A | B>(c: C): void => (c.kind == "a" ? a(c) : b(c))

Of course in this simple example C could just be defined a type alias, but in the code this is extracted from I need C to be generic and then this doesn't work anymore.

[jack-williams]

@rolfvandekrol That is probably related to this: #21483, or at least it suffers from the same issues.

[dartess]

So few answers and reactions to such an old and simple problem... :С

A similar problem: https://stackoverflow.com/questions/63594468/type-guard-dont-work-with-generic-params