In Typescript: Is there a way to initialize an object or array inline (with a certain interface type)?

Question

In Typescript I want to do something like this:

interface SomeType {
 name: string
}
useState([]:SomeType[]); // This creates an empty array of SomeType inline & calls a function called useState

or even

function getArrayOfSomeType():SomeType[] {
  return []:SomeType[];
}

Other cases might involve the same concept but with a map/dictionary:

// Maybe there is a better way to declare maps?
interface MapOfSomeTypes {
 [key:string]: SomeType
}

function getMapOfTypes():MapOfSomeTypes {
  return {} as MapOfSometypes; // or perhaps {}:MapOfSomeTypes
}

This is to get around having to declare these before and then returning them or passing them to another function when all I need is an empty version of them (like for react-hooks).

Is there any problem with useState([]); or return [] correspondingly? — zerkms
– zerkms, Commented Nov 8, 2019 at 19:39
I am probably missing something but with react's useState it looks like: const [elementOfSomeType, functionThatTakesSomeType] = useState({}); and I can't figure out how to declare the types for them without initializing an empty object outside of this line with type:SomeType. Since useState is likely taking a generic type I think I need to declare that somewhere to make all the types match. — KJason
– KJason, Commented Nov 8, 2019 at 19:43
I'm not sure how code from your comment correlates with your original question. Hence, is there a reason to declare a type explicitly? Cannot TS infer it for you? — zerkms
– zerkms, Commented Nov 8, 2019 at 19:45
See above, just clarified shortly after you replied but it can't really infer it because I believe useState is taking a generic. — KJason
– KJason, Commented Nov 8, 2019 at 19:47
I did see it, my question still stands: any problem with NOT declaring the type explicitly and using type inference? — zerkms
– zerkms, Commented Nov 8, 2019 at 19:47

zerkms · Accepted Answer · 2019-11-08 19:47:17Z

2

No need to declare them explicitly - typescript would infer it for you, hence

interface SomeType {
 name: string
}
useState([]);

function getArrayOfSomeType():SomeType[] {
  return [];
}

interface MapOfSomeTypes {
 [key:string]: SomeType
}

function getMapOfTypes():MapOfSomeTypes {
  return {};
}

answered Nov 8, 2019 at 19:47

zerkms

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8 Comments

Ruan Mendes Over a year ago

This example is not not inferring it, you're creating a function that basically casts an inferred any[] to the return type of the function?

zerkms Over a year ago

@JuanMendes how can you tell that?

Ruan Mendes Over a year ago

If you don't explicitly say SomeType[] as the return value of getArrayOfSomeType(), it will be inferred to be any[]. However, your function casts it to SomeType[] when it returns it?

zerkms Over a year ago

"If you don't explicitly say SomeType[] as the return value, it will be any[]," --- how can you tell that? What makes you think it's not inferred based on the argument or return value type? Or at least that it's not inferred as [] type?

Ruan Mendes Over a year ago

It's inferred by the caller of getArrayOfSomeType, but within the function, it's just as if you had instead done return [] as SomeType[], so you're just hiding the casting behind the function, but it's still casting

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