Typescript dynamically creating types from object literal

Issue

Sorry this could well be a duplicate as I see lots of similar questions just haven't yet managed to apply those examples to my situation...

Given I call my function loadStuff() like so:

loadStuff({
  dataExample1: loadDataExample1Fn(),
  dataExample2: loadDataExample2Fn(),
});

Where:

• There could be 1 to many dynamic keys to data load calls
• The example loadDataExample1Fn has signature () => LoadedData<DataExample1>
• The example loadDataExample2Fn has signature () => LoadedData<DataExample2>

I want the return type to be:

LoadedData<{ dataExample1: DataExample1, dataExample2: DataExample2 }>

Edit: Added Typescript playground example

Anyone able to point me in the right direction in getting typescript support on the returned result?

Solution

You can indeed! To do this you'll need to make your function generic, and then generate a return type from the inferred generic type using conditional types

const loadStuff = <L extends { [x: string]: LoadedData<any> }>(
  loadedDataItems: L
): LoadedData<{
  [k in keyof L]: L[k] extends LoadedData<infer D> ? D : never;
}> => {
  // Stuff
};

Answered By - Ben Wainwright