typescript restrict count of object's properties

Question

Is it possible to restrict the count of object properties, say I want to restrict object have only one string propery (with any name), I can do:

{[index: stri         


        

Answer 1

Because this question has been marked as a duplicate of this one, let me answer here.

Check if a type is a union

/**
 * @see https://stackoverflow.com/questions/53953814/typescript-check-if-a-type-is-a-union/53955431
 */
type IsSingleton =
  [T] extends [UnionToIntersection]
    ? true
    : false

/**
 * @author https://stackoverflow.com/users/2887218/jcalz
 * @see https://stackoverflow.com/a/50375286/10325032
 */
type UnionToIntersection =
  (Union extends any
    ? (argument: Union) => void
    : never
  ) extends (argument: infer Intersection) => void
    ? Intersection
  : never;

Allow only singleton types

type SingletonOnly =
  IsSingleton extends true
    ? T
    : never

Restrict a function to accept only a singleton type

declare function foo(s: SingletonOnly): void

declare const singleton: 'foo';
foo(singleton);

declare const union: "foo" | "bar";
foo(union); // Compile-time error

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