Types not narrowing with discriminated union

[hearty helm]

It seems like this simple type narrowing doesn’t work as it should:

[tepid plaza]

.selrond#0

Playground Link

Preview:```ts
const address = {
id: '123',
fieldConfig: {
type: 'address'
},
value: {
city: 'asdf'
}
}

const num = {
id: '3456',
fieldConfig: {
type: 'number'
},
value: 123
}

type Field = typeof address | typeof num

const fn = (field: Field) {
...```

You can choose specific lines to embed by selecting them before copying the link.

[floral bramble]

you have a few problems here

first of all, if you look at the types of the fieldConfig properties you'll see they are merely { type: string }. there's no type-level difference between address.fieldConfig and num.fieldConfig

you might've meant to use as const on those values

however, even if you do that your narrowing attempt will not work

discriminated unions must follow a specific pattern of having a literal-typed key at the top level

the only extra narrowing rule you get with discriminated unions is that x may be narrowed when you check x.field, if field is a discriminant property

when you check field.fieldConfig.type, only field.fieldConfig will ever be narrowed, not field itself. it's not "deep"

if you can move type up to the top level it'll work

[hearty helm]

Wow, thanks!
Is this behavior documented somewhere? There are lots of examples on TS docs, but nothing explicit about the mechanism ike you’ve described

@floral bramble

when you check field.fieldConfig.type, only field.fieldConfig will ever be narrowed, not field itself. it's not "deep"

I’ve tried to access the fieldConfig based on what you’ve said to see if it’s narrowed, but it doesn’t seem to be

[tepid plaza]

@hearty helm Here's a shortened URL of your playground link! You can remove the full link from your message.

.selrond#0

Playground Link

Preview:```ts
const address = {
id: '123',
fieldConfig: {
type: 'address',
address: 'true'
},
value: {
city: 'asdf'
}
}

const num = {
id: '3456',
fieldConfig: {
type: 'number',
num: true
},
value: 123
}

type Field = typeof address | typeof num
...```

You can choose specific lines to embed by selecting them before copying the link.

[floral bramble]

the documentation page i linked to above says this:

When every type in a union contains a common property with literal types, TypeScript considers that to be a discriminated union, and can narrow out the members of the union.

In this case, kind was that common property (which is what’s considered a discriminant property of Shape). Checking whether the kind property was "circle" got rid of every type in Shape that didn’t have a kind property with the type "circle". That narrowed shape down to the type Circle.

[hearty helm]

doesn’t mention that the type must not be nested though

[floral bramble]

right, it says what does work (and that's all that works). i do agree it could spell this out more clearly

[floral bramble]

you forgot the as consts again. whenever you encounter errors like this a good first move is to hover over the relevant variables (where you'll see fieldConfig is just always { type: string } there)

[hearty helm]

I mean I’d assume common property inside a type can very well be a nested one, so yeah it’s not clear at all from the docs:

type A = {
  property: 'a',
  nested: {
    property: {
      property: 'b' 
    }
  }
}

[floral bramble]

ah yeah i don't think "contains" implies recursion or anything there. maybe it's clearer when i swap it for "has":

When every type in a union has a common property with literal types

[hearty helm]

btw, do you know whether this is a limitation of the TS language of sorts, or an intended behaviour?
On a first look it seems like it should work this way 🤔

[floral bramble]

i believe it's simple for performance reasons. computing every possible permutation at every depth of an object would be expensive/slow. i'll see if i can dig up any source for that

https://github.com/microsoft/TypeScript/issues/18758 is a relevant issue

there was an attempt to implement it that sadly didn't land: https://github.com/microsoft/TypeScript/pull/38839