Typescript 错误？ Function 接受 Object 定义为接受类型 A 和 B 的联合，其中 B 扩展 A

Question

This might be a bug or me not understanding union types. Take a look at this code:

TS Playground

interface A {
  a?: string;
  b?: boolean;
}

interface B extends A{
  x: boolean;
  y: string;
}

function printCoord(pt: A | B) {
  console.log("The coordinate's x value is " + pt.a);
  console.log("The coordinate's y value is " + pt.b);
}
printCoord({ a: "asdf", b: true });

printCoord({a: "asdf", x: false});

The last line should not compile, because the presence of x enforces type B , but y is missing so it should actually be invalid.

Answer 1

The union type is used to declare a type that is either one (or even both).

example:

interface A {
  a1: string;
  a2: string;
}

interface B {
  b1: string;
  b2: string;
}

const e1: A|B = {a1: "1", a2:"1"}  // valid
const e2: A|B = {b1: "1", b2:"1"}  // valid
const e3: A|B = {a1: "1", b1:"1"}  // invalid

In your example, you are trying to assign {a: "asdf", x: false} to A | B A | B . since type A accepts pretty much anything (even {} ) then {a: "asdf", x: false} can be assigned to A | B A | B .

The reason it doesn't throw an error about the fact that there is a B property there is because A | B A | B allows the value to be a mix of both. It has to be at least A or B, but can "inspire" to be both.

that's why this is also valid

printCoord({ a: "asdf" });  // valid