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Is there any way to check if a type includes 'undefined' type?

type a = boolean | undefined;

type v = a extends undefined ? 'yes' : 'no'

I tried with extends, but it doesn't work.

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  • 5
    And your name is undefined, that all makes sense now. :D Commented Sep 13, 2021 at 13:53
  • 1
    @RyanLe interface undefined extends Human Commented Sep 13, 2021 at 14:13

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Try to use conditional type with extra generic parameter

type a = boolean | undefined;

type IsUndefined<T> = undefined extends T ? true : false

type Result = IsUndefined<a> // true

type Result2 = IsUndefined<number | string> // false

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When conditional types act on a generic type, they become distributive when given a union type (distributive-conditional-types). For example, take the following.

If we plug a union type into IsUndefined, then the conditional type will be applied to each member of that union.

type IsUndefined<T> = T extends undefined ? true : false

type Result = IsUndefined<string> | IsUndefined<undefined> // boolean

IsUndefined<string> returns false IsUndefined<undefined> returns true

Hence, Result is boolean because a union of true and false is a boolean.

But, if you put undefined before extends then conditional type will be applied to the whole union

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3 Comments

Thanks for the answer. Can you please explain why it works?
you just had them backwards, it's undefined extends a instead of a extends undefined
@undefined sure, give me a minute

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