Typescript: "type?" is optional in an array, while "undefined" isn't

Question

Two signatures I thought were the same are behaving differently from each other:

type FooType = [ string, number? ]
const fooAr = [ "foo" ] as FooType // compiles

type BarType = [ string, number | undefined ]

// Conversion of type '[string]' to type 'BarType' may be a mistake because neither type sufficiently overlaps with the other.
const barVar = [ "bar" ] as BarType

I wouldn't mind except I'm inferring the type from a library and can't change the signature. Is this how Typescript is supposed to behave?

== Edit

Was hoping it followed the object type behavior:

type FooType = { foo: string, bar: string | undefined }
const fooOjb = { foo: "abc" } as FooType // compiles

Yep. Not existing and existing with a value of undefined are different. — k.tten
– k.tten, Commented Oct 5, 2022 at 22:35
About your update: that only works because it's a type assertion: tsplay.dev/mZXnEw. See this question for the differences and nuances. — k.tten
– k.tten, Commented Oct 5, 2022 at 22:44

Michael Brenndoerfer · Accepted Answer · 2022-10-05 22:38:58Z

2

? and undefined are not interchangable.

type FooType = [ string, number? ] means that the array can look like this:

["hello"] or ["hello", 123]

type BarType = [ string, number | undefined ] means that the array can look like this:

["hello", undefined] or ["hello", 123]

answered Oct 5, 2022 at 22:38

Michael Brenndoerfer

4,2062 gold badges48 silver badges64 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

k.tten Over a year ago

Important to note that this behavior is also seen in objects, but you can turn on exactOptionalPropertyTypes to change that.

Michael Brenndoerfer Over a year ago

Totally right. Thanks for pointing it out @caTS

Sean Over a year ago

Dang! Exactly why I thought it'd behave that way, thanks

davetapley · Accepted Answer · 2022-10-05 22:37:52Z

1

FooType is using an optional, where as BarType is using undefined.

For why they are different see here.

answered Oct 5, 2022 at 22:37

davetapley

18.1k15 gold badges65 silver badges93 bronze badges

Collectives™ on Stack Overflow

Typescript: "type?" is optional in an array, while "undefined" isn't

2 Answers 2

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related