typescript interface: optional but not undefined

Question

How can I describe type like

key is optional, but if it existed, it cannot be undefined.

At the first time, I'm using Partial to make all properties of T optional.

interface A {
  a: number;
}

var data: A = {
  a: 1,
}

var update1: Partial<A> = {

}

var update2: Partial<A> = {
  a: undefined
}

var result1: A = {
  ...data,
  ...update1,
} // { a: 1 }

var result2: A = {
  ...data,
  ...update2,
} // { a: undefined }

The problem here is that result2 is not implementing interface A in runtime, but typescript never complains about it. Is it bug or feature? I guess typescript not working well with spread operator...

The goal is distinguish these two variable using typescript!


var data: T = {
  a: 1,
  b: 2,
}

var optional1: MyPartial<T> = { // Totally OK

}

var optional2: MyPartial<T> = { // OK
  a: 3,
}

var optional3: MyPartial<T> = { // OK
  b: 4,
}

var optional4: MyPartial<T> = { // OK
  a: 3,
  b: 4,
}

var undef1: MyPartial<T> = { // typescript must throw error
  a: undefined,
}

var undef2: MyPartial<T> = { // typescript must throw error
  a: 3,
  b: undefined
}

...

Check this out TypeScript playground example.

I don't believe it's possible, unfortunately, due to this (still) open issue: github.com/Microsoft/TypeScript/issues/13195; there are some workarounds depending on how exactly you really want to use it, but a general solution seems impossible at the moment. :-/ — dtanabe
– dtanabe, Commented Jan 18, 2019 at 23:02
this should really be supported. I like using Partial<T> for redux action creators so I don't need to include all of the props on the T object (e.g., setting part of a User type while registering), but this allows me to potentially set the wrong property type :( — Tunn
– Tunn, Commented Jan 19, 2020 at 22:07

cdosborn · Accepted Answer · 2021-10-01 16:46:58Z

14

Support now exists in 4.4.0 via --exactOptionalPropertyTypes

So by default, TypeScript doesn’t distinguish between a present property with the value undefined and a missing property ...

In TypeScript 4.4, the new flag --exactOptionalPropertyTypes specifies that optional property types should be interpreted exactly as written, meaning that | undefined is not added to the type:

https://devblogs.microsoft.com/typescript/announcing-typescript-4-4/#exact-optional-property-types

answered Oct 1, 2021 at 16:46

cdosborn

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Community · Accepted Answer · 2020-06-23 09:33:03Z

3

As @dtanabe mentioned, there isn't really a good solution here until TypeScript support (if they decide to). And main problem from doing it in user land is that in a var def (i.e. const x: Foo = ...) we do not have access to the type of the value. If you are OK changing your code from using spread to using a function you can do something like this:

interface A {
  a: number;
}

type EnsurePartial<TTarget, TUpdate extends keyof TTarget> =
  undefined extends TTarget[TUpdate]
  ? never
  : { [Key in TUpdate]: TTarget[Key] };

declare function update<T>(propsToUpdate: EnsurePartial<A, T>): void

update({}) // OK
update({ a: 42 }) // OK
update({ a: undefined }) // error
update({ c: undefined }) // error

edited Jun 23, 2020 at 9:33

CommunityBot

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answered Jan 21, 2019 at 9:02

Dmitriy

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1 Comment

Acid Coder Over a year ago

there is error in this code: Type 'T' does not satisfy the constraint '"a"'.

Acid Coder · Accepted Answer · 2020-08-13 18:29:25Z

1

There is an error in Dmitriy answer, here is the fixed version

interface A {
  a: number;
  b: boolean
}

type PartialWithoutUndefined<M,T> = M extends T
    ?  undefined extends M[keyof M]
            ? never
            : M
    : never


declare function update<M>(propsToUpdate: PartialWithoutUndefined<M,Partial<A>>): void

update({}) // OK
update({ a: 42 }) // OK
update({ a: undefined }) // error
update({ c: undefined }) // error

playground

weakness: it is impossible to type anything as undefined even if that is your intention

edited Aug 13, 2020 at 18:29

answered Aug 13, 2020 at 18:16

Acid Coder

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1 Comment

Acid Coder Over a year ago

@DanielTate what do you mean?

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