Why is TS erroring about a simple conditional type in a function?

[hasty flame]

https://www.typescriptlang.org/play/?#code/KYOwrgtgBAasAWBLAxgG2AFQJ4AdhQG8AoKUqZAQwCcAaEsgIwHsKAXOgXyKMRFeCoAzCsnwBhaoXqlWuYAC5YCFOmx4AdJSoBuaVBAUICqAGdWVXgHNdZKAHd4wYKhOLwEBgN1cefAcNEoACEWVilbWTxFOCQ0TDl1ZjYbMgMjRTMLEGs9eAoTAGUKRBdFZiZ0ChBvbkEwEGRWRCYQKAgKAGtgGJVgAB4ABXyTYAATHri1fGAAD34QUZMlWNU5AD4ACgA3ZUm5RSGTEfHd1bwafUNjTKsASj0D4bGJs+m50EXl3qnNSQB+KASKhQR5HZ6neJ4KCzeafF6Q4CJUJQAEhNgg-TAHbA4i2RCCKDbCFTKAAXlJXz2Gi0t3CtjIyBaZigW0UQLJdPp9MixnhPy0dC5XLSwEFQtsDicpSgABYxVyOCkuQB6ZVQACiVCoTGBiBMelsqqgJIA5ECTVA9fomGFhohLAYGOgoKwmC65FATYdjnyPTCPktfdT-oDJKCfcS-e8FoHIxokmFUci3FiBCaDWQqMBWGAqK0tkqoFw8QSiSsEWSKUHEQnabihYyQMzWcFkRT6+KZPtKa8kWx5Z2RQPxXlCsVpeYwKKM6RFTOoEbNdrdfrxUbTebLUsQDaoHaHRQnfhXe6oV6nidyyT-TGewjfsCAezw+Cr1HYbG3-HkUn0Snsem4pZjmeYsoWxZkKw8DanYmKwUuOobCaACqICzHgjRjCyEKnsAJq3N4QA

In the code, I am literally typing the function to say "If the passed in type is a, return x, and if the passed in type is b, return y" and then in the body of the code, I'm literally doing exactly the same thing, saying "If the passed in type is a, return this value of type x, and if the passed in type is b, return this value of type y"

[wet yew]

@hasty flame Here's a shortened URL of your playground link! You can remove the full link from your message.

lielmus#0

Playground Link

Preview:```ts
enum VehicleType {
car,
boat,
}

interface Car {
type: VehicleType.car
name: string
wheels: number
}

interface Boat {
type: VehicleType.boat
name: string
hasSails: boolean
}

function makeVehicle<
Pas
...```

You can choose specific lines to embed by selecting them before copying the link.

[mossy tapir]

You cannot narrow generic, it's not always safe.

You either:

• (Imo the better way) split up the function into makeCar and makeBoat.
• Use mapped type if you really want to have just one function.

[hasty flame]

But I mean, I'm literally narrowing it with a discriminated union

Just seems superweird that the compiller doesn't allow it

And I mean the logic in the conditional type PassedVehicleType extends VehicleType.car ? Car exactly overlaps with the implementation if (vehicleType == VehicleType.car) return car: Car ...

[mossy tapir]

You can narrow the value of a generic (vehicleType) but you cannot narrow the generic itself (PassedVehicleType)

Just because vehicleType === VehicleType.car, doesn't mean PassedVehicleType has to be Car, it could still be a wider type (Car | Boat), or it could still be a narrower type (Car & { extraProperty: string }).

In this case the return is safe even though you can't narrow it, but it's not safe in general, and that's why it's not supported.

[hasty flame]

Thanks for explaining that. I've rewritten it but its not quite right. I don't really understand the Mapped Type documentation, its so abstract to me. Any tips?

[wet yew]

@hasty flame Here's a shortened URL of your playground link! You can remove the full link from your message.

lielmus#0

Playground Link

Preview:```ts
enum VehicleType {
car,
boat,
}

interface Car {
type: VehicleType.car
name: string
wheels: number
}

interface Boat {
type: VehicleType.boat
name: string
hasSails: boolean
}

type VehicleTypes = {
[VehicleType.car]:
...```

You can choose specific lines to embed by selecting them before copying the link.

[mossy tapir]

You can do something like this:

[wet yew]

nonspicyburrito#0

Playground Link

Preview:```ts
enum VehicleType {
car,
boat,
}

interface Car {
type: VehicleType.car
name: string
wheels: number
}

interface Boat {
type: VehicleType.boat
name: string
hasSails: boolean
}

type VehicleTypes = {
[VehicleType.car]: C
...```

You can choose specific lines to embed by selecting them before copying the link.