TypeScript: ReturnType -> 如何设置 function 返回的 function 的返回类型

Question

I am using graphql codegen to generate the types for my GraphQL codebase. A sample code of the core sdk generated looks like this

export function getSdk(client: GraphQLClient, withWrapper: SdkFunctionWrapper = defaultWrapper) {
  return {
   myCustomFunction(...): Promise<MyFuncReturn>
 }
}

getSdk returns an object of all the API's I have defined for my GraphQL Server.

I am trying to use ReturnType syntax to match the return value of myCustomFunction ie MyFuncReturn in other parts of the code. Reason being, many of my queries / output response objects are incomplete (I dont want all the fields for all the queries) and hence I cant typecast it to the complete response object defined in the schema.

I want to leverage ReturnType to such that my other parts of the code can just leverage the returntype of the function rather than knowing what object / type is being returned, something like:

mytestCode {

    callCoreSdk(): ReturnType<getSdk().myCustomFunc>
}

Is this possible and allowed. Tried the above and getting error. Also, could not locate examples where I can use ReturnType for a func inside a func.

Answer 1

Your syntax is a bit off, but it can be done. To get the type of getSdk you need to use the typeof operator ( typeof getSdk ). Then can use ReturnType to get the return type of getSdk ( ReturnType<typeof getSdk> ). To get the type of a property in the return type you can use an index type query ( ReturnType<typeof getSdk>['myCustomFunction'] )

export function getSdk() {
  return {
    myCustomFunction(a: string): Promise<MyFuncReturn> { return null! }
 }
}

interface mytestCode {
    callCoreSdk(): ReturnType<typeof getSdk>['myCustomFunction']
}

declare var x: mytestCode;
x.callCoreSdk()("")

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