Need some way to propagate optional fields from `satisfies` to type it is used on

Suggestion

Give some way to do satisfies-like check checking for type with optional properties and modyfy left-handside type of the check to make these prperties available for access with type never.

Solution 1

Use extends keyword similar to satisfies, but with propagating optional fields with type never to computed type it is used on:

import type { Equal, Expect } from '@type-challenges/utils'

const obj1 = { a: 1, c: 3 } as const   satisfies   Partial<Record<"a" | "b" | "c", number>>;
const obj2 = { a: 1, c: 3 } as const    extends    Partial<Record<"a" | "b" | "c", number>>;
//                                    ^^^^^^^^^^^

type cases = [
  Expect<Equal<typeof obj1, {
    readonly a: 1;
    readonly c: 3;
  }>>,
  Expect<Equal<typeof obj2, {
    readonly a: 1,
    readonly b: never, // <<<<<<<<
    readonly c: 3,
  }>>,
]

Solution 2

Introduce new special type missing that is equvalent of optional never but without appearing in keyof and make satisfies set this type to missing optional properties and propagete it on type it is used on.

import type { Equal, Expect } from '@type-challenges/utils'

const obj = { a: 1, c: 3 } as const satisfies Partial<Record<"a" | "b" | "c", number>>;

type cases = [
  Expect<Equal<typeof obj, {
    readonly a: 1;
    readonly b: missing,
    readonly c: 3;
  }>>,
  Expect<Equal<keyof typeof obj, "a" | "c">>,
  Expect<Equal<keyof { x: number; y: missing }, "x">>,
  Expect<Equal<keyof { x: number; y: never }, "x" | "y">>,
]

const smth1: { x: number, y: missing } = { x: 0 } // Fine
const smth2: { x: number, y?: never } = { x: 0 } // Fine
const smth3: { x: number, y: never } = { x: 0 } // Currently an error

By the way, this type may be useful in many other cases.

🔍 Search Terms

satisfies Partial never

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

When I use satisfies I actually expect that I can use on an object all kinds of property access that were available on type I'm using for the assertion. However I came up to a situation where I can't. At first I thought it's a bug, but then I realized that "fixing" it will break keyof functionality, so I desided that it would be incorrect to modify satisfies behaviour, so I suggest to use another keyword for this functionality.

Playground

type SomeKeys = "a" | "b" | "c"

type Smth = Partial<Record<SomeKeys, number>>

const obj1 = { a: 1, c: 3 } as const satisfies Smth;

const obj2 = { a: 1, c: 3 } as {
  readonly a: 1,
  readonly b: never,
  readonly c: 3,
} satisfies Smth;

const keys: SomeKeys[] = ["a", "b", "c"]

function f(x: Smth) {
  console.log(keys.map(key => x[key])); // Fine
}

f(obj1); // Fine
console.log(keys.map(key => obj1[key])); // Error

f(obj2); // Fine
console.log(keys.map(key => obj2[key])); // Fine

type K1 = keyof typeof obj1 // Fine: "a" | "c"
type K2 = keyof typeof obj2 // Oops: "a" | "b" | "c"

satisfies Smth on obj1 checkes that key b can be used to index it, anyway as type of obj1 doesn't include key b the code doesn'ty compile with

Element implicitly has an 'any' type because expression of type 'SomeKeys' can't be used to index type '{ readonly a: 1; readonly c: 3; }'.
Property 'b' does not exist on type '{ readonly a: 1; readonly c: 3; }'.(7053)

📃 Motivating Example

In this example it's possible to use type normally without satisfies but I didn't want to overcomplicate an example. I hope it's idea is clear.

Playground

import React from 'react'

type Pages = "home" | "search" | "news" | "profile"

const pages: Pages[] = ["home", "search", "news"]

interface User {
  name: string;
  isRegistered: boolean;
  canSearch: boolean;
}

const canOpenPage = { // Not listed pages are available for everyone
  profile: user => user.isRegistered,
  search: user => user.canSearch,
} as const satisfies Partial<Record<Pages, (user: User) => boolean>>;

function Navigation({ currentUser }: { currentUser: User }) {
  return (
    <nav>
      {pages.map(page => canOpenPage[page]?.(currentUser) === false || (
        <a href={`/${encodeURIComponent(page)}`}>{page}</a>)
      )}
    </nav>
  )
}

Should not have error

Element implicitly has an 'any' type because expression of type 'Pages' can't be used to index type '{ readonly profile: (user: User) => boolean; readonly search: (user: User) => boolean; }'.
Property 'home' does not exist on type '{ readonly profile: (user: User) => boolean; readonly search: (user: User) => boolean; }'.

💻 Use Cases

Usages with Partial<Record<...>> and keys enumeration seem the most likely for me.

The issue names no repository files or tests. Reproduce the satisfies and Partial<Record<...>> examples in the linked TypeScript Playgrounds, then trace the type-checker behavior for indexed access and keyof. Done means an accepted design and implementation for the proposed optional-field behavior, with regression tests covering the documented examples.