Typescript initialise a type with initial/ default value

Question

Is it possible to create a Type with value in it.

eg:

type Animal = {
    kind : "animal"
    Legs : number,
    CanFly: boolean
}

const monkey: Animal = { Legs: 4, CanFly: false};  //In this line, clients has to initialize the same value `kind : "animal"`

I would to create an attribute called kind and use that to infer the object and make decisions. However, in the next line, i would expect the clients to pass the same value back in all the initializations. Otherwisem TS compiler would complain `Property 'kind' is missing in type'.

Is there a way to default it without the clients have to pass it back?

Answer 1

TypeScript is a structurally-typed language. Meaning when you defined a type or interface you defined a shape other objects must conform to. And assigning defaults within TS types is not possible.

type Animal = {
  kind : "animal"
  Legs : number,
  CanFly: boolean
}

I assume you are on a recent version of TS since your kind is a string literal type, "animal", and it can only ever be that string literal. And you have to supply kind: "animal" when you create objects of Animal shape.

Now, if you want to implement discriminated unions, you might choose the following approach (perhaps animal is a little to generic in this case):

type Snake = {
  kind: "snake"
  Legs : number,
  CanFly: boolean
}

type Dolphin = {
  kind: "dolphin",
  Legs: number,
  CanFly: boolean
}

type Monkey = {
  kind: "monkey",
  Legs: number,
  CanFly: boolean
}

Then a discriminated union:

type Animal = Snake | Dolphin | Monkey 

To reiterate, TS does not support defaults in a type declaration. TS types define the shapes of things and if an object, O, doesn't conform to a shape, T, well then O is not a T.

Answer 2

You can create a factory method:

type Animal = {
    kind : "animal"
    Legs : number,
    CanFly: boolean
}

function createAnimal(parameter: Omit<Animal, 'kind'>): Animal {
    return {
        kind: 'animal',
        ...parameter,
    };
}

const monkey = createAnimal({ Legs: 4, CanFly: false});