Help understanding difference between "Extract<keyof MyObject, string>" and"keyof MyObject & string" | TypeScript Community | Page 1

timber lantern Aug 12, 2024, 3:44 PM

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Hi,

Can someone help me understand why one of my types below works fine, but the other causes an error?
My understanding is these should be functionally the same, but I'm clearly missing something.

The error I receive on the broken one is:

Type 'keyof MyObject & string' cannot be used to index type '{ [K in keyof MyObject as K extends string ? K : never]: { column: K; op: "=" | "!=" | "<" | "<=" | ">" | ">="; value: MyObject[K]; }; }'

Both are filtering the keys of MyObject to just the string keys, but TS is moaning at me for one of them.

Any help understanding this would be great please.

export type WhereTypeWORKING<MyObject> = {
  [K in Extract<keyof MyObject, string>]: {
    column: K;
    op: "=" | "!=" | "<" | "<=" | ">" | ">=";
    value: MyObject[K];
  };
}[Extract<keyof MyObject, string>];

export type WhereTypeBROKEN<MyObject> = {
  [K in keyof MyObject as K extends string ? K : never]: {
    column: K;
    op: "=" | "!=" | "<" | "<=" | ">" | ">=";
    value: MyObject[K];
  };
}[keyof MyObject & string];

An example showing the same output from both methods of extracting string keys

type Table = {
  id: number;
  name: string;
  42: boolean;
  [Symbol.iterator]: () => Iterator<any>;
};

// both equate to "id" | "name"
type test = keyof Table & string
type test2 = Extract<keyof Table, string>

Many thanks in advance!

warm iris Aug 12, 2024, 3:46 PM

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this is only an issue because MyObject is generic here, so ts can't evaluate the expressions.

timber lantern Aug 12, 2024, 3:46 PM

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does it not infer its an object from the keyof?

warm iris Aug 12, 2024, 3:47 PM

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no, not at all, but that's not the issue

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adding an extends object clause on the generic doesn't change the result

timber lantern Aug 12, 2024, 3:47 PM

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if i tried to pass it a non-object it would complain though?

warm iris Aug 12, 2024, 3:48 PM

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no, it'd work just fine

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object vs non-object is completely irrelevant here

timber lantern Aug 12, 2024, 3:48 PM

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ooh ok

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I'm not sure I understand why the bottom example equates the same, but isn't allowed though, sorry

warm iris Aug 12, 2024, 3:50 PM

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for any type, Extract<keyof T, string> and keyof T & string would evalutate to the same result
but ts can't evaluate those when T doesn't exist yet

in the working example, the keys from the mapped type and the indexer are trivially equivalent
you could use keyof MyObject & string instead in both positions and it'd also work, because again, trivially equivalent
but that's not the case with the remapping clause

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if MyObject wasn't generic, ts could evaluate the mapping and see that the types are actually equivalent:

gleaming kelpBOT Aug 12, 2024, 3:51 PM

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that_guy977#0

Playground Link

Preview:```ts
type MyObject = {
id: number
name: string
42: boolean
[Symbol.iterator]: () => Iterator<any>
}

export type WhereTypeWORKING = {
[K in Extract<keyof MyObject, string>]: {
column: K
op: "=" | "!=" | "<" | "<=" | ">" | ">="
value: MyObject[
...```

warm iris Aug 12, 2024, 3:51 PM

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but because MyObject doesn't exist yet, ts can't do that

timber lantern Aug 12, 2024, 3:51 PM

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with this bit:

or any type, Extract<keyof T, string> and keyof T & string would evalutate to the same result
but ts can't evaluate those when T doesn't exist yet

does that mean there is a possible value of T that wouldn't satisfy both?

warm iris Aug 12, 2024, 3:52 PM

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no

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ts just doesn't have the logic to say Extract<K, string> and K & string and K extends string ? K : never yield the same result

timber lantern Aug 12, 2024, 3:54 PM

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oooh ok, thanks!
Do you know anywhere I can read more on this?
I've been through about 4 different typescript courses now, finish each one feeling confident, then immediatly hit a wall when implementing into my codebase lol

warm iris Aug 12, 2024, 3:54 PM

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#Help understanding difference between "Extract<keyof MyObject, string>" and"keyof MyObject & string"