how to type function within object inline?

Question

given a type like:

type SomeFunctionType = (arg: string) => string

it it possible to apply this type to a function within an object inline? Basically apply the type SomeFunction to someFunction below.

const someObject = {
  someFunction: (arg) => arg
}

The only ways I've found is either to have the function outside the object:

const someFunction: SomeFunctionType = (arg) => arg

const someObject = {
  someFunction
}

or to type the object:

const someObject: { someFunction: SomeFunctionType } = {
  someFunction: (arg) => arg
}

but it feels like there should be a shorter way.

Alex Wayne · Accepted Answer · 2020-02-06 19:06:34Z

2

Type casting any individual value is done with as.

const someObject = {
  someFunction: ((arg => arg) as SomeFunctionType)
}

Or fully type the function so it matches your intended type:

const someObject = {
  someFunction: (arg: string): string => arg
}

But most of the time you probably do want provide an interface at the object level. But it's hard to advise on that with only a contrived example.

answered Feb 6, 2020 at 19:06

Alex Wayne

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