An example in Typescript for setting an initial/default value for a data type

Question

An example in Typescript for setting an initial/default value for a data type

Can you create a Type with a default value included?

For example:

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

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

I want to include an attribute named kind and use it to deduce the object and make decisions. However, in the subsequent line, I expect clients to provide the same value for all initializations. Otherwise, TypeScript compiler will generate an error 'Property 'kind' is missing in type'.

Is there a way to set a default value without requiring clients to pass it back?

typescript

Answer 1

Answer №1

When working with TypeScript, it's important to remember that it is a structurally-typed language. This means that when you create a `type` or `interface`, you are defining a specific shape that other objects must adhere to. It's worth noting that assigning defaults within TypeScript types is not supported.

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

If your type definition includes properties like `kind: "animal"` as a string literal type, this indicates that only that exact value can be assigned to it. In this case, any object created using the `Animal` shape must have `kind: "animal"` specified.

To implement discriminated unions in TypeScript, you could consider creating more specific types instead of a generic one such as 'Animal'. For instance:

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

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

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

These specific types can then be used to create a discriminated union like so:

type Animal = Snake | Dolphin | Monkey

To summarize, TypeScript focuses on defining shapes rather than providing default values within type declarations. If an object does not align with a specific shape defined by a TypeScript type, it cannot be considered as that type.

Answer 2

When working with TypeScript, it's important to remember that it is a structurally-typed language. This means that when you create a `type` or `interface`, you are defining a specific shape that other objects must adhere to. It's worth noting that assigning defaults within TypeScript types is not supported.

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

If your type definition includes properties like `kind: "animal"` as a string literal type, this indicates that only that exact value can be assigned to it. In this case, any object created using the `Animal` shape must have `kind: "animal"` specified.

To implement discriminated unions in TypeScript, you could consider creating more specific types instead of a generic one such as 'Animal'. For instance:

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

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

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

These specific types can then be used to create a discriminated union like so:

type Animal = Snake | Dolphin | Monkey

To summarize, TypeScript focuses on defining shapes rather than providing default values within type declarations. If an object does not align with a specific shape defined by a TypeScript type, it cannot be considered as that type.

Answer 3

Answer №2

To simplify the code, you can implement a factory method:

type Creature = {
    type : "creature"
    Legs : number,
    CanFly: boolean
}

function createCreature(properties: Omit<Creature, 'type'>): Creature {
    return {
        type: 'creature',
        ...properties,
    };
}

const dragon = createCreature({ Legs: 6, CanFly: true});

Answer 4

To simplify the code, you can implement a factory method:

type Creature = {
    type : "creature"
    Legs : number,
    CanFly: boolean
}

function createCreature(properties: Omit<Creature, 'type'>): Creature {
    return {
        type: 'creature',
        ...properties,
    };
}

const dragon = createCreature({ Legs: 6, CanFly: true});

Answer 5

Answer №3

To implement a class instead of an object literal, you can use the following approach:

export interface Animal {
    type: string;
     Legs : number;
     CanFly: boolean;
   }
   
   export class AnimalInfo implements Animal {
       type: string;
       Legs : number;
       CanFly: boolean;

       static create () {
          return new AnimalInfo({});
       }
   
       public constructor (animal: Partial<Animal>) {
          this.type = animal.type || "default type";
          this.Legs= animal.Legs || -1;
          this.CanFly = animal.CanFly || false;
       }
   }

const dog = AnimalInfo.create();

const cat = new AnimalInfo({type: "domestic", Legs: 4, CanFly: false});

Answer 6

To implement a class instead of an object literal, you can use the following approach:

export interface Animal {
    type: string;
     Legs : number;
     CanFly: boolean;
   }
   
   export class AnimalInfo implements Animal {
       type: string;
       Legs : number;
       CanFly: boolean;

       static create () {
          return new AnimalInfo({});
       }
   
       public constructor (animal: Partial<Animal>) {
          this.type = animal.type || "default type";
          this.Legs= animal.Legs || -1;
          this.CanFly = animal.CanFly || false;
       }
   }

const dog = AnimalInfo.create();

const cat = new AnimalInfo({type: "domestic", Legs: 4, CanFly: false});

An example in Typescript for setting an initial/default value for a data type

Answer №1

Answer №2

Answer №3

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