Why isn't it working T extends Record<symbol,unknown>-CodePudding

type Test<T extends Record<symbol, unknown>> = T;

type Result = Test<{
  a: string;
}>;

I made this T extends Record<symbol, unknown> , Test only accept index type with symbol key,why Test<{a:string}> no false alarms ?

CodePudding user response：

Typescript interfaces are "open", meaning that additional properties can be present on an object outside of its known interface, and this is not a compile time error.

For this reason, a value of type {a: string} is a valid value for Record<symbol, unknown>.

You can verify this with the following code that compiles perfectly fine:

const obj: {a: string} = {a: "foo"};
const record: Record<symbol, unknown> = obj;

CodePudding user response：

If you want it to be invalid you can intersect it with another Record whose values are never:

type Test<T extends Record<symbol, unknown> & Record<string | number, never>> = T;;

type Result = Test<{ // ! error, string/number keys should be type never
  a: string;
}>;

Works well enough

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