windows 10 pro office 2019 pro office 365 pro windows 10 home windows 10 enterprise office 2019 home and business office 2016 pro windows 10 education visio 2019 microsoft project 2019 microsoft project 2016 visio professional 2016 windows server 2012 windows server 2016 windows server 2019 Betriebssysteme office software windows server instagram takipçi instagram beğeni instagram görüntüleme instagram otomatik beğeni facebook beğeni facebook sayfa beğenisi facebook takipçi twitter takipçi twitter beğeni twitter retweet youtube izlenme youtube abone instagram

Typescript object fields keys generically

Asked By: Anonymous

I have a data structure that contains various optional fields with different types that have to be fetched individually and formatted. The formatting functions take concrete values.

I tried doing something like the following minimal working example, but it errors with with Property 'value' does not exist on type 'Data[P]'.

I understand why the problem happens, but I want to make this work. I quick hack would be to cast data[idx][key] as Optional<T>, but can this be done without type casts?

// In my code this is more complex, a simple `T | undefined` doesn't work.
type Optional<T> = { kind: 'none' } | { kind: 'value', value: T};

interface Data {
  name: Optional<string>;
  id: Optional<number>;

function computeIndex(): number {
  // Some logic here
  return 0;

// Format field `key` of `data` using `formatter`.
// Use default `def` if the value is `none`.
function printField<
  P extends keyof Data,
  T extends Data[P] extends Optional<infer V> ? V : never
>(data: Data[], key: P, formatter: (val: T) => string, def: T): string {
  const idx = computeIndex();

  const field = data[idx][key];

  switch (field.kind) {
    case 'none':
      return formatter(def);

    case 'value':
      return formatter(field.value);


Answered By: Anonymous

I don’t believe there is a cast-free solution at present.

Ultimately, TS seems to have trouble inferring Optional<string | number> from Optional<string> | Optional<number> in the field declaration, so we give it the extra nudge in the form of the Optional<T> cast.

Is this a hack? If you view it as working around the TS checker’s inference limitations, maybe. I would agree it was a hack if it opened the door to mistyping values, or if the result didn’t work as well as desired

But the end result works as well as a "non-hack" solution, in that it correctly type-checks formatter given key. And I don’t see how the cast could result in the wrong type of value making it through.

techinplanet staff

Windows 10 Kaufen Windows 10 Pro Office 2019 Kaufen Office 365 Lizenz Windows 10 Home Lizenz Office 2019 Home Business Kaufen windows office 365 satın al follower kaufen instagram follower kaufen porno