Typescript initialise a type with initial/ default value

Issue

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?

Solution

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});

Answered By - HTN