typescript is not able to Inference my type

Question

As I filtered vehicles based on isCar so Ts should infer it as an array of Cars but TS infer it as Foo[]. How can I have a conditional type based on isCar??

interface Vehicle {
  driveName: string
}

interface Bicycle extends Vehicle {
  isCar: false,
  saddleColor: string
}

interface Car extends Vehicle {
  isCar: true
  plate: string
}


type Foo = Bicycle | Car


const vehicles: Foo[] = [
  {
    driveName: "foo",
    isCar: true,
    plate: "1111"
  },
  {
    driveName: "bar",
    isCar: false,
    saddleColor: "red"
  }
]


const c = vehicles.filter(({ isCar }) => isCar) // typeof c is not Car

my code

I found these questions but was not able to find an answer to my problem

Typescript Interface - Possible to make "one or the other" properties required?

Typescript asserting types if they have one property true or false

Please include your code in the question. So people can see your problem without leaving SO. — Eldar, Oct 27 '22 at 06:41
@Eldar I'm new to stackoverflow and not able to post image. follow the link I put in post — mohammadizanloo55, Oct 27 '22 at 06:56
Does one these posts: [Why can't TypeScript infer type from filtered arrays?](https://stackoverflow.com/questions/62032214/why-cant-typescript-infer-type-from-filtered-arrays) or [Way to tell TypeScript compiler Array.prototype.filter removes certain types from an array?](https://stackoverflow.com/questions/43010737/way-to-tell-typescript-compiler-array-prototype-filter-removes-certain-types-fro) , answer your question? — Palladium02, Oct 27 '22 at 07:21
@mohammadizanloo55 please always include minimum reproducible example in your question, otherwise your question will probably be closed — captain-yossarian from Ukraine, Oct 27 '22 at 07:29

score 1 · Accepted Answer · answered Oct 27 '22 at 07:27

You need to use custom typeguard:

interface Vehicle {
  driveName: string
}

interface Bicycle extends Vehicle {
  isCar: false,
  saddleColor: string
}

interface Car extends Vehicle {
  isCar: true
  plate: string
}


type Foo = Bicycle | Car


const vehicles: Foo[] = [
  {
    driveName: "foo",
    isCar: true,
    plate: "1111"
  },
  {
    driveName: "bar",
    isCar: false,
    saddleColor: "red"
  }
]

const isCar = (vehicle: Foo): vehicle is Car => vehicle.isCar

const c = vehicles.filter(isCar) // typeof c is not Car

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typescript is not able to Inference my type

1 Answers1