keyof to produce a runtime array

[markboyall]

Put simply, I'd like a variant of keyof which produces the keys at runtime. E.g.,

interface Foo {
   prop1: number;
   prop2: string;
}

const x = keyof Foo;

In this case x should be ["prop1", "prop2"].

I've seen a few cases where people have tried to use keyof in somewhat-similar fashions, but they all wanted the actual keys present in the object at runtime. I however very explicitly want to ignore any properties present in any hypothetical runtime Foo objects that are not present in the interface.

Should permit generic parameters as the type.

[krryan]

Perhaps this could be generalized to turning a union of literal types (as keyof Foo is, being 'prop1' | 'prop2') into an array containing each member of the union once (['prop1', 'prop2']). I have a number of places I would like to see that, particularly for testing (have a test iterate over the array to make sure that each value supposedly acceptable to the union does, in fact, produce an appropriate result).

[zpdDG4gta8XKpMCd]

in what order should these properties go?

[krryan]

I for one don't care; alphabetical, as-declared, officially-undefined, I just want it to be complete.

[zpdDG4gta8XKpMCd]

i think it should be a codefix (refactoring) rather than a runtime facility:

// before                                

const x = keyof Foo;
          ^^^^^^^^^

// codefix                                
                      ______________________________________________
const x = keyof Foo; |                                              |
          ^^^^^^^^^ <   Replace with all names of `Foo` properties  |
                     |______________________________________________|

// after
const x = ["prop1", "prop2"];

[krryan]

It's better than nothing but it definitely doesn't do everything I would have liked. For example, in my testing example, the entire point is ensuring complete coverage of the union for the test case. If a new item is added to the union, that array is still a valid array of items from that union, but it is no longer "complete"—some values from the union are not represented in the array.

[zpdDG4gta8XKpMCd]

on the other hand, the fact that there is no way to know the order of names in which they will be listed is disturbing

in a meanwhile there is a hack:

const mySample : Foo {
   prop1: 0,
   prop2: ''
};
function toKeysBySample<T>(value: T): (keyof T)[] {
    return Object.keys(value);
}
const fooPropNames = toKeysBySample(mySample);

although it's not perfect either: #12253 (comment)

[krryan]

That hack does nothing for the case of a union that isn't derived from keyof though. Personally, I am only concerned about order in a minority of cases; lacking knowledge of the order in this case wouldn't bother me. The type of the resulting "array" could also be a tuple, which would capture the order.

[Igorbek]

Should permit generic parameters as the type.

that wouldn't that easy

function keys<T>(): (keyof T)[] {
  return keyof T;
}
console.log(keys<{ a: number; }>());
console.log(keys<{ c: bool; }>());

what would it emit? Consider:

• no object of type T to pass to Object.keys
• T is generic, cannot emit static value at keyof T
• would need to introduce ambient parameter
  • pass it only if it is used?
  • need to resolve on call-site?
  • generics can be propagated through call stack
  • generics can be constructed from other generics (T & G[])
  • .d.ts !

[zpdDG4gta8XKpMCd]

a better hack that is almost to the point (yet has problems with unions)

interface Foo {
    kind: 'a';
    value: number;
}
interface Baz {
    kind: 'b';
    text: string;
}
type Bar = Foo | Baz;
type Names<T> = { [P in keyof T]: P; }
function toKeysBySample<T>(names: Names<T>): (keyof T)[] {
    return Object.values(names);
}
toKeysBySample<Bar>({ kind: 'kind', text: 'text' });

[zpdDG4gta8XKpMCd]

@Igorbek has a good point, if we remember that any type driven emit is taboo

[mhegazy]

As noted by @Igorbek and @Aleksey-Bykov, type directed emit is something that violate the TS goals of a fully erasable type system.

Moreover, a keyof T is not guaranteed to be the complete list at runt time you would get from Object.keys since the TS types are not exact/final.

For both reasons, this is out of scope for the TypeScript project at the time being.

[markboyall]

The second reason is the entire reason I wanted this feature. I don't want the complete runtime list. I want strictly and only the ones listed in the interface.

As for the first, I guess I just don't see why that should be a goal at all.