Typescript dynamically creating types from object literal

Question

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?

Can you create a typescript playground that includes all the related types? — Ramesh Reddy
– Ramesh Reddy, Commented Mar 5, 2022 at 14:53

Ben Wainwright · Accepted Answer · 2022-03-05 15:22:19Z

1

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 Mar 5, 2022 at 15:22

Ben Wainwright

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1 Comment

Ed . Over a year ago

Great - thanks very much. I don't pretend to completely follow this but will do some reading. 👍

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