TypeScript keyof of nested object type

Question

I want to get a union of all the keys under on in this object type inferred from a configuration object:

type Config = {
initial: string;
states: {
    idle: {
        on: {
          START: string;
        };
        effect(): void;
    };
    running: {
        on: {
            PAUSE: string;
        };
        effect(): void;
    };
    paused: {
        initial: string;
        states: {
            frozen: {
                on: {
                    HEAT: string;
                };
            };
        };
        on: {
          RESET: string;
        };
    };
};

}

Note that the configuration can have nested on keys under states. Right now I can get the first level keys using:

type KeysOfTransition<Obj> = Obj extends Record<PropertyKey, any> ? keyof Obj : never;
type TransitionKeys = KeysOfTransition<Config["states"][keyof Config["states"]]["on"]>
// "START" | "PAUSE" | "RESET"

But I cannot get HEAT in the union, which is nested. Any ideas?

Is the second lv on always under the second lv states?

zixiCat
– zixiCat

2021-03-26 03:03:57 +00:00
Commented Mar 26, 2021 at 3:03 — zixiCat
– zixiCat, Commented Mar 26, 2021 at 3:03

jcalz · Accepted Answer · 2021-03-26 03:05:29Z

8

There may well be edge cases, but here's an implementation that works for your example:

type KeysUnder<T, K extends PropertyKey> =
  T extends object ? {
    [P in keyof T]-?: (P extends K ? keyof T[P] : never) | KeysUnder<T[P], K>
  }[keyof T] : never;
    
type ConfigOnKeys = KeysUnder<Config, "on">;
// type ConfigOnKeys = "START" | "PAUSE" | "RESET" | "HEAT"

The idea is to use a recursive conditional type to drill down into object properties. To get KeysUnder<T, K>, for an object T, we look at each property key P. If P matches K (which will be "on" in your use case), then you want the keys of its the property value type T[P]. The recursive bit is that we also join all these keys to KeysUnder<T[P], K>.

Playground link to code

answered Mar 26, 2021 at 3:05

jcalz

339k29 gold badges452 silver badges449 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

cassiozen Over a year ago

This works perfectly - I'm getting a "Type instantiation is excessively deep and possibly infinite." when feeding ConfigOnKeys to other functions, but I can figure it out.

jcalz Over a year ago

Yes, recursive conditional types do have issues, and possible infinite regress is one of them. There are techniques to try to prevent that from happening, but without a minimal reproducible example it's hard to know how to advise.

Mohammad Jawad Barati Nov 23, 2024 at 10:52

Just out of curiosity, should not TS be able to infer the type automatically?

jcalz Nov 23, 2024 at 20:15

Infer what type automatically where? This is a 3-year-old answer and I don't see anywhere in this q/a where inference would be happening. I'd ask you to clarify but this probably isn't the place for it. You might want to open your own question post if you have a question.

Collectives™ on Stack Overflow

TypeScript keyof of nested object type

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related