Code Quiz

Broken Mutable Mapped Type

A mapped type meant to strip readonly actually keeps it — spot the missing modifier.

Codetypescript

type Mutable<T> = {
  readonly [P in keyof T]: T[P];
};

interface Config {
  readonly host: string;
  readonly port: number;
}

const c: Mutable<Config> = { host: "localhost", port: 8080 };
c.port = 9090; // intended to be allowed

The `Mutable<T>` type is supposed to make all properties writable, but assigning to `c.port` still errors. What is the bug?

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