Question 201471 in Javascript Question posted in
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Javascript – why typescript's structure typing is inconsistent?

we know that typescript applies structure typing as below example shows:

interface Vector {
   x: number;
   y: number;
}

interface NamedVector {
   x: number;
   y: number;
   name: string;
}

function calculateLength(v: Vector) {
   return Math.sqrt(v.x * v.x + v.y * v.y);
}

const v: NamedVector = { x: 3, y: 4, name: 'zee' };
calculateLength(v);   // compile OK, result is 5

It allowed calculateLength to be called with a NamedVector because its structure was compatible with Vector.

But in term of assignment, it doesn’t use structure typing anymore:

const v: Vector = { x: 3, y: 4, name: 'Zee' };  // compile error, 'name' does not exist in type 'Vector'

according to the definition of structure typing, { x: 3, y: 4, name: 'Zee' } is also compatible with Vector, so why structure typing doesn’t work here?

and what is the Utility Types I can use to describe a type that must contains x and y fields and also some other fields so I can do:

const v: XXX<Vector> = { x: 3, y: 4, name: 'Zee' };
Tags:

3

Answers


  1. 0

    To answer your second question

    interface Vector {
   x: number;
   y: number;
}
const v: Vector & {[x:string]: any}= { x: 3, y: 4, name: 'Zee' };

    TypescriptPlayground

    The first question is not specific enough to answer, in my opinion.

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  2. 0

    I cannot think of a single use-case where this would be make sense but the simplest way to down-cast is to, well, cast

    const v = { x: 3, y: 4, name: 'Zee' } as Vector;

    Of course you won’t be able to access v.name so again, this seems a pointless exercise.

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  3. 0

    You should not use excess property when use Object literal. So you should use Custom Genernics not to use excess property.

    interface Vector {
   x: number;
   y: number;
}

interface NamedVector {
   x: number;
   y: number;
   name: string;
}

function calculateLength(v: Vector) {
   return Math.sqrt(v.x * v.x + v.y * v.y);
}

type Optional<T, N extends keyof T> = Partial<Pick<T, N>> & Omit<T, N>;

const v: Optional<NamedVector, "name"> = { x: 3, y: 4, name: 'Zee' };
calculateLength(v);

    ref: https://www.typescriptlang.org/docs/handbook/type-compatibility.html#starting-out

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