Type inference for literal types with Object.fromEntries

fromEntries-with-literal-types.ts

type Primitive =
  | boolean
  | number
  | string
  | bigint
  | symbol
  | null
  | undefined;

type Narrowable =
  | Primitive
  | object
  | {};

type Entry<K extends PropertyKey, V> = [K, V];

/**
 * @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;

type FromEntries<T extends Entry<K, V>, K extends PropertyKey, V extends Narrowable> =
  UnionToIntersection<
    T extends [infer Key, infer Value]
      ? Key extends PropertyKey
        ? Record<Key, Value>
        : never
      : never
  >

function fromEntries<T extends Entry<K, V>, K extends PropertyKey, V extends Narrowable>(
  entries: Iterable<T>,
): FromEntries<T, K, V> {
  return [...entries].reduce(
    (accumulator, [key, value]) =>
      Object.assign(accumulator, {
        [key.toString()]: value,
      }),
    {} as FromEntries<T, K, V>,
  );
}

fromEntries([
  ['foo', 1],
  ['bar', 2]
]); // #ExpectType { foo: 1, bar: 2}

Good observation. As to the results: I think you'd have to explicitly provide a type argument to xs.map<> in order to get the results we want. We would have to somehow map over the tuple and feed that type to .map.

type Entries<T> = {
  [K in keyof T]:
    T[K] extends Record<any, infer Value>
      ? Value extends PropertyKey
        ? [Value, T[K]]
        : never
      : never
};

type T1 = Entries<typeof xs>; // $ExpectType readonly [ ["foo", { readonly key: 'foo' }], ["bar", { readonly key: 'bar' }] ]

A similar case for UnionToIntersection for @maktarsis:

interface Path<T extends string> {
  path: T;
}

declare function fn<T extends Path<U>, U extends string>(paths: readonly T[]): Result<T>;

type Result<T extends Path<string>> =
  UnionToIntersection<
    T extends Path<infer Pathname>
      ? { [index in Pathname]: Pathname }
      : never
  >;

fn([{ path: 'home' }, { path: 'about' }]); // $ExpectType { home: 'home', about: 'about' }

const paths = [
  { path: 'home' },
  { path: 'about' },
] as const;

fn(paths); // $ExpectType { home: 'home', about: 'about' }

/**
 * @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;

Use this definition to overload Object.fromEntries:

type Primitive =
  | boolean
  | number
  | string
  | bigint
  | symbol
  | null
  | undefined;

type Narrowable =
  | Primitive
  | object
  | {};

type Entry<K extends PropertyKey, V> = [K, V];

/**
 * @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;

type FromEntries<T extends Entry<K, V>, K extends PropertyKey, V extends Narrowable> =
  UnionToIntersection<
    T extends [infer Key, infer Value]
      ? Key extends PropertyKey
        ? { [Property in Key]: Value }
        : never
      : never
  >

declare global {
  interface ObjectConstructor {
    fromEntries<T extends Entry<K, V>, K extends PropertyKey, V extends Narrowable>(entries: Iterable<T>): FromEntries<T, K, V>
  }
}

Object.fromEntries([
  ['foo', 1],
  ['bar', 2]
]); // #ExpectType { foo: 1, bar: 2}