Strictly typed values alongside message with variables

Problem Description

Currently if you use MessageDescriptors directly

i18n._({
  id: "uxV9Xq",
  message: "Hey {firstName}!",
  values: { firstname: "Alice" },
})

Values will be typed as

values?: Record<string, unknown>

Proposed Solution

From the context it would be possible to infer the variables and strictly type the values.

type ExtractVariables<T extends string> =
  T extends `${string}{${infer Variable}}${infer Rest}`
    ? Variable | ExtractVariables<Rest>
    : never;

type HasVariables<T extends string> = ExtractVariables<T> extends never
  ? false
  : true;

type MessageDescriptor<T extends string = string> = {
  id: string;
  comment?: string;
  message?: T;
} & (HasVariables<T> extends true
  ? {
      values: {
        [K in ExtractVariables<T>]: unknown;
      };
    }
  : {
      values?: Record<string, unknown>;
    });

// ...

declare class I18n extends EventEmitter<Events> {
  // ...

  _<T extends string = string>(descriptor: MessageDescriptor<T>): string;

  // ...
}

And in the above example that would be

values: {
  firstName: unknown;
}

This would prevent missing values or adding extraordinary keys to variable values. It helps detect that firstname was incorrectly capitalized and missing

Alternatives Considered

Keep types loose and unsafe

Additional Context

No response

Start by locating the TypeScript definitions for MessageDescriptor and the I18n._ method shown in the proposal. Trace how message descriptors and values are currently typed, then verify that variables inferred from the message require matching value keys while messages without variables retain optional values.