类型在Typescript中是结构性的。 对象文字的问题

Question

From book Basarat - TypeScript Deep Dive

interface Point2D {
    x: number;
    y: number;
}
interface Point3D {
    x: number;
    y: number;
    z: number;
}
var point2D: Point2D = { x: 0, y: 10 }
var point3D: Point3D = { x: 0, y: 10, z: 20 }
function iTakePoint2D(point: Point2D) { /* do something */ }

iTakePoint2D(point2D); // exact match okay
iTakePoint2D(point3D); // extra information okay
iTakePoint2D({ x: 0 }); // Error: missing information `y`

I replace this line:

iTakePoint2D(point3D); // extra information okay

with

iTakePoint2D({ x: 0, y: 10, z: 20 }); 

and I have an Error. Why is now extra information not okay?

Answer 1

Object literals always undergo excess property checks when they are assigned to other types or they are passed as an argument to a function.

https://www.typescriptlang.org/docs/handbook/interfaces.html#excess-property-checks