How come this doesn't work?

[odd crow]

U9G#0670

Playground Link

Preview:```ts
import type {
Equal,
Expect,
NotAny,
} from "@type-challenges/utils"

const elems = [{a: "apple"}, {a: "orange", b: 1}]

const {a, ...rest} = elems[Math.floor(Math.random())]

if (a === "orange") {
type Y = typeof rest.b
type X = Expect<Equal<Y, number>>
...```

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[rose solstice]

number | undefined is not the same as number

those two types are not compatible

so Equal returns false

and since Expect is expecting true, you're getting the error

[rose solstice]

the reason why it's | undefined is because you are taking one element from [{ a: 'apple' }, { a: 'orange', b: 1 }]
so b might not be defined in some cases
it's not guaranteed it exists

[heavy wadi]

I understand why it fails, I'm asking why typescript can't specify

[heavy wadi]

it is though? since we used a === 'orange' then that would mean the rest has to have b

[rose solstice]

yeah, I get that
however, that's not how typescript work
you are checking the a property, but then you try to access the b property, that you haven't checked

(even tho your check on the a property also influences the resul on the b property)

you should create 2 different types
and use type predicates to tell them appart

https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

or use a discrimiated union, even better
https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

@heavy wadi

[heavy wadi]

I don't want to make my own types, this data comes from a json

idk, im just stuck in somewhere that even the compiler cant help

[rose solstice]

then type predicates are the best choice

also, since you used object destructuring, those two a and rest variabels are now behaving independenlty for the compiler

[odd crow]

Ascor8522#7606

Playground Link

Preview:```ts
import type {
Equal,
Expect,
} from "@type-challenges/utils"

interface Apple {
a: "apple"
}

interface Orange {
a: "orange"
b: 1
}

type Fruit = Apple | Orange

const elems: Fruit[] = [
{a: "apple"},
{a: "orange", b: 1},
]
const fruit = elems[Math.floor(Math.random())]
...```

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

[rose solstice]

@heavy wadi

also, you need to change Equal<Y, number> to Equal<Y, 1>

or define Orange.b as number (and not as 1)

[heavy wadi]

yeah I see the problem

[marsh bronze]

if (a === 'orange') {
    type Y = typeof rest.b
    type X = Expect<Equal<NonNullable<Y>, number>>
}

X = true

[rose solstice]

only if the array is declared as const (const elems = [{ a: 'apple' }, { a: 'orange', b: 1 }] as const;)