How to check if union type can be undefined or not using extends?

[undone apex]

Given a type type MyType = string | undefined how do I type check if the type can be undefined? I tried the following but it doesn't work. Any idea whats the correct way to do this?

type IsUndefined = MyType extends undefined ? 'yes' : 'no'
// no

[humble violet]

krimson#7581

Playground Link

Preview:```ts
type MyType = string | undefined

type IsUndefined = MyType extends undefined
? "yes"
: "no"
// ^?```

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[fickle sinew]

@undone apex MyType extends undefined checks if your type is a subtype of undefined - you'd want the reverse: undefined extends MyType ? 'yes' : 'no'

This checks if undefined is a subtype of your type.

[undone apex]

oh duh that makes sense! Thank you

!solved

[fickle sinew]

!resolved