TypeScript中基于已有Interface创建新Interface为何失效？

Great question—let’s break down the two key issues preventing your code from working as expected:

1. Mapped syntax isn’t supported in interface declarations #

TypeScript’s mapped type pattern ([K in keyof A]) is designed to work with type aliases, not interfaces. Interfaces have strict structural rules and don’t allow dynamic key iteration syntax directly. This is why your original interface B declaration fails.

2. You’re using the wrong boolean type #

Boolean (uppercase) refers to the JavaScript wrapper object type, which is almost never what you want for property types. The primitive boolean type you need for true/false values is lowercase boolean.

The Fixed Solution #

Replace the interface for B with a type alias using the correct mapped syntax and primitive type:

interface A {
  x: string;
  y: string;
  z: string;
  // maybe later something more
}

// This dynamically generates the type you want!
type B = {
  [K in keyof A]: boolean;
};

Now B will automatically mirror all keys from A with boolean types. If you add new properties to A later, B will update to include them without any extra code—exactly what you were aiming for.

Bonus: If You Really Need an Interface #

While a type alias is the cleanest approach here, if you must use an interface for specific reasons, you can extend the mapped type like this:

interface B extends { [K in keyof A]: boolean } {}

But this is unnecessary in most cases—the type alias is simpler and achieves the same result.

内容的提问来源于stack exchange，提问作者Rozrewolwerowany rewolwer