Create an object type without a certain key | TypeScript Community | Page 1

waxen finch Jul 27, 2023, 8:46 AM

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Hi! Im trying to create a type which is an object and can hold any key except "data"

type ObjectWithoutData = {
            [key: string]: any;
        } & {
            data?: never;
        };

const x: ObjectWithoutData = {
    test: "hello",
    data: "test" // ERRORS which is what I want!
};

let y: any = {}

y["test"] = x.data; // Doesnt error :/

I've got something like this but you can still access x.data, now I know that the assignment works because it's optional but if you make it not optional then you need to include it which I also dont want.... is there any solution to this?

soft grove Jul 27, 2023, 8:54 AM

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@waxen finch https://www.typescriptlang.org/docs/handbook/utility-types.html#omittype-keys

Documentation - Utility Types

Types which are globally included in TypeScript

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type ObjectWithoutData = Omit<{
  [key: string]: any;
}, "data">;

waxen finch Jul 27, 2023, 8:54 AM

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doesnt work 😄

soft grove Jul 27, 2023, 8:55 AM

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waxen finch doesnt work 😄

what error do you get?

waxen finch Jul 27, 2023, 8:55 AM

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Well none which is the problem, you can use data without any errors

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        type ObjectWithoutData = Omit<
            {
                [key: string]: any;
            },
            "data"
        >;

        const x: ObjectWithoutData = {
            test: "hello",
            data: "test",
        };

        let y: any = {};
        y["test"] = x.data;

#

no errors

slate flicker Jul 27, 2023, 9:10 AM

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It's fundamentally impossible in TS to write a type that disallows a property entirely

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That data?: never is a clever hack that makes it impossible to assign it an object that has a data property because there is no way to create such a value.

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But all you are really saying is that data is a property of the bottom type never

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so yes it only blocks assignment to the type, but reading from it works fine and you even get a type that's assignable to anything (after you remove the undefined)

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(and purely looking at type-land you can even still assign to this property:)

maiden swallowBOT Jul 27, 2023, 9:15 AM

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angryzor#0

Playground Link

Preview:```ts
import process from "process"

const x: {foo: never} = {foo: process.exit()}```

slate flicker Jul 27, 2023, 9:17 AM

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(personally I really don't like this hack to be honest but it gets spread everywhere)

neon cave Jul 27, 2023, 9:23 AM

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You can use a generic identity function and validate it doesn't have data property.

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type DisallowData<T> = {
    [K in keyof T]: K extends 'data' ? never : T[K]
}

const disallowData = <T>(obj: DisallowData<T>) => obj

disallowData({})
disallowData({ foo: 123 })
disallowData({ data: 456 })
disallowData({ foo: 123, data: 456 })

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!ts

maiden swallowBOT Jul 27, 2023, 9:25 AM

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type DisallowData<T> = {
    [K in keyof T]: K extends 'data' ? never : T[K]
}

const disallowData = <T>(obj: DisallowData<T>) => obj

disallowData({})
disallowData({ foo: 123 })
disallowData({ data: 456 })
//             ^^^^
// Type 'number' is not assignable to type 'never'.
disallowData({ foo: 123, data: 456 })
//                       ^^^^
// Type 'number' is not assignable to type 'never'.

neon cave Jul 27, 2023, 9:25 AM

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But even this can be worked around.

slate flicker Jul 27, 2023, 9:28 AM

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Thought of that too but that will only work of they allow all result objects to be inferred

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Because you can't define a result type that disallows data:

maiden swallowBOT Jul 27, 2023, 9:28 AM

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angryzor#0

Playground Link

Preview:```ts
type DisallowData<T> = {
[K in keyof T]: K extends "data" ? never : T[K]
}

type F = DisallowData<{[key: string]: any}>
// ^?```

slate flicker Jul 27, 2023, 9:29 AM

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(from being read)

neon cave Jul 27, 2023, 9:30 AM

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Yeah you have to use the disallowData function, can't use the naked type.

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So instead of:

const foo: ? = { ... }

You have to do:

const foo = disallowData({ ... })

(And even that can still be worked around)

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Depends on why you want to disallow data, there might be better ways.

waxen finch Jul 27, 2023, 12:51 PM

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I wanted to have something like this if it makes sense

type ObjectWithoutData = {
            [key: string]: any;
        } & {
            data?: never;
        };

type ObjectWithData = Record<string, any>;

// response from api either contains an object with a data section or the object is missing the data section
const myApiData: ObjectWithoutData | ObjectWithData = {}

// imagine this to be a  like an app store
const store: Record<string, any> = {}

// type guard
if (myApiData.data) {
    // I want to be able to access data here cleanly
    store["data"] = myApiData.data // should work
} else {
    // i dont want to be able to access data here and if i do so get an error.
    store["something2"] = myApiData.data // should error
}

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and no, I dont know how the data looks like, I only know that it either has a data property in the object or it does not and I kinda want that typed

neon cave Jul 27, 2023, 12:57 PM

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What other keys does it have besides the data?

waxen finch Jul 27, 2023, 12:57 PM

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not known at compile time

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because otherwise i would just create two types where data is missing in one of them but yea :/

neon cave Jul 27, 2023, 12:58 PM

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If you don’t know anything about the object keys, then data is no different from any other key.

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Just use myApiData: object, and you already have to do 'data' in myApiData check before you use it.

waxen finch Jul 27, 2023, 1:00 PM

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still wont prevent anyone from trying to access data in the else

neon cave Jul 27, 2023, 1:00 PM

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It will

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declare const api: object

if ('data' in api) {
  api.data
} else {
  api.data
}

#

!ts

maiden swallowBOT Jul 27, 2023, 1:01 PM

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declare const api: object

if ('data' in api) {
  api.data
} else {
  api.data
//    ^^^^
// Property 'data' does not exist on type 'object'.
}

waxen finch Jul 27, 2023, 1:01 PM

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oh wow xd

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well that solves it then i guess

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didnt even have to try anything fancy

neon cave Jul 27, 2023, 1:02 PM

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You problem was that

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{ [k: string]: any } is the wrong type to begin with.

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That type means “any string is a valid property” and that’s not true.

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Because any string is a valid property, you can access anything, not just api.data but also api.beef and api.salsa.

waxen finch Jul 27, 2023, 1:05 PM

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but i still find it a bit weird that this doesnt work

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declare const api: Record<string, any>;

if ('data' in api) {
  api.data
} else {
  api.data
}

#

!ts

maiden swallowBOT Jul 27, 2023, 1:05 PM

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declare const api: Record<string, any>;

if ('data' in api) {
  api.data
} else {
  api.data
}

neon cave Jul 27, 2023, 1:05 PM

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It works, any key is a valid property no matter if it passes the check or not, it’s still valid.

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What you really want is “I don’t know if any key exists at all so force me to check it first”

waxen finch Jul 27, 2023, 1:09 PM

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how would i solve this?

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const store: Record<string, any> = {}
declare const api: object

if ('data' in api) {
  store["data"] = api.data[0];
} else {
}

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!ts

maiden swallowBOT Jul 27, 2023, 1:09 PM

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const store: Record<string, any> = {}
declare const api: object

if ('data' in api) {
  store["data"] = api.data[0];
//                ^^^^^^^^
// 'api.data' is of type 'unknown'.
} else {
}

waxen finch Jul 27, 2023, 1:09 PM

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just as any?

neon cave Jul 27, 2023, 1:10 PM

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You can’t access api.data[0] unless you know it’s an array, so check if it’s an array using Array.isArray.

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Didn’t you say you don’t know anything about the API? How do you know data might exist and it might be an array and you want the first element?

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Because “I don’t know anything about its shape” is very different from “I know its shape I’m just not sure if it will be there”

waxen finch Jul 27, 2023, 1:14 PM

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I know the rough shape of data if it exists

neon cave Jul 27, 2023, 1:14 PM

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Describe it.

waxen finch Jul 27, 2023, 1:14 PM

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In the end it’s just an array of Record<string, any> 😂

neon cave Jul 27, 2023, 1:15 PM

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Well you certainly know more than that, otherwise how are you going to use api.data[0]?

waxen finch Jul 27, 2023, 1:17 PM

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Using [0] was just an example, basically I wanted to know if there’s a way to type it a bit more even when it’s an object

neon cave Jul 27, 2023, 1:17 PM

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Then use the same narrowing of key in object or Array.isArray.

waxen finch Jul 27, 2023, 1:18 PM

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Okay 👍

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Thanks

neon cave Jul 27, 2023, 1:20 PM

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But really, if you know api.data.salsa might be there, you can do way better:

declare const api: {
  data?: {
    salsa?: string
  }
}

if (api.data?.salsa) {
  api.data.salsa
} else {
  api.data.salsa
}

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!ts

maiden swallowBOT Jul 27, 2023, 1:20 PM

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declare const api: {
  data?: {
    salsa?: string
  }
}

if (api.data?.salsa) {
  api.data.salsa
} else {
  api.data.salsa
//^^^^^^^^
// 'api.data' is possibly 'undefined'.
}

#Create an object type without a certain key