How to narrow an union type

[dreamy lodge]

Say I have a type

type Transaction = { kind: 'income', customer: string } | { kind: 'expense', vendor: string };

and I need to narrow it down to just the income kind or the expense kind.

type NarrowDown<Type, Key, Value> = /* ??? implementation */

type IncomeTransaction = NarrowDown<Transaction, 'kind', 'income'> // { kind: 'income', customer: string }

is this possible?

[tropic summit]

Ascor8522#7606

Playground Link

Preview:```ts
type Transaction =
| {kind: "income"; customer: string}
| {kind: "expense"; vendor: string}

type NarrowDown<
Type,
Key extends keyof Type,
Value extends Type[Key]

= Type extends any
? Type[Key] extends Value
? Type
: never
: never

type IncomeTransaction = NarrowDown<
Transaction,
"kind",
"income"

...```

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

[maiden cedar]

you can easily do that using conditional types

https://www.typescriptlang.org/docs/handbook/2/conditional-types.html

the thing is, you're dealing with a union (Transaction)

so you need to distribute that union first

see the "Distributive Conditional Types" section on the same page

that's why there is a "weird" extends any check first

@dreamy lodge

[prime mist]

what

im so confused

isn't this just Extract

why do you need to distribute for this

[maiden cedar]

true, it's Extract

[dreamy lodge]

oh so Extract<Transaction, { kind: 'income' }>?

[prime mist]

yes

[dreamy lodge]

makes sense thaanks

[prime mist]

for future reference your union is a discriminated union, useful for other things

and as mentioned, this is extraction, not narrowing. narrowing is figuring out the type of a value

[dreamy lodge]

can I close this or will it automatically

!close