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: string]: any}

to restrict the type of property, but can one restrict also the count of properties?

Answer 1

There are many answers to this question on Stackoverflow (including this detailed one), but none of them worked for my situation, which is similar to the one posted here.

Problem

I have a function that takes an object. I want it to throw a Compilation Error (Typescript) if the passed object doesn't have exactly one key. e.g.

f({}); // Must error here, as it has less than one key!
f({ x: 5 });
f({ x: 5, y : 6 }); // Must error here, as it has more than one key!

---

Solution

Using the popular UnionToIntersection and IsUnion, I achieved it via the following utility function.

type SingleKey<T> = IsUnion<keyof T> extends true ? never : {} extends T ? never : T;

Full Code:

// From https://stackoverflow.com/a/50375286
type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never;

// From: https://stackoverflow.com/a/53955431
type IsUnion<T> = [T] extends [UnionToIntersection<T>] ? false : true;

// Here we come!
type SingleKey<T> = IsUnion<keyof T> extends true ? never : {} extends T ? never : T;

// Usage:
function f<T extends Record<string, any>>(obj: SingleKey<T>) {
    console.log({ obj });
}

f({}); // errors here!
f({ x: 5 });
f({ x: 5, y : 6 }); // errors here!

Playground Link

Answer 2

Most likely no. The best solution that comes to my mind is wrapping an Object (or Map) with you custom class with methods set(key: string, val: any) and get(key: string) that can disallow adding new items to the underlying collection under certain circumstances.

Answer 3

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> =
  [T] extends [UnionToIntersection<T>]
    ? true
    : false

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

Allow only singleton types

type SingletonOnly<T> =
  IsSingleton<T> extends true
    ? T
    : never

Restrict a function to accept only a singleton type

declare function foo<K extends string>(s: SingletonOnly<K>): void

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

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

TypeScript Playground