Determining Specific Type in Switch Case

While I have come across similar questions on this topic, I couldn't find one that addresses my specific case or that I fully comprehend. Let me explain the situation and share the code:

1. I have a React component that receives a store as a prop. 2. This store has a "tab" property which can be either "points", "segments", or "tags". 3. Additionally, the store contains arrays for points, segments, or tags, each of type Array. 4. Within my component, I need a variable named "selectedEntity" that could belong to any of these types. To achieve this, I initially declared it without specifying the type and then attempted to assign the appropriate value and type within a switch statement. However, TypeScript is raising complaints. Even after assignments and type assertions, TypeScript continues to flag 'selectedEntity' as being of type any.

Code:

const TagCustomization: React.FC<PropsType> = ({ store, curr }) => {

let selectedEntities;
let thisEntity;
let thisTag;

useEffect(() => {
  switch (store.tab) {
    case "points":
      selectedEntities = store.points.filter(
        (point) => point.selected,
      ) as Array<Tpoint>;
      thisEntity = selectedEntities[curr] as Tpoint;
      thisTag = store.tags.find(
        (tag) => tag.id == thisEntity.id,
      );
      break;
   // The same issue occurs in the cases below
    case "segments":
      selectedEntities = store.segments.filter(
        (seg) => seg.selected,
      ) as Array<Tsegment>;
      thisEntity = selectedEntities[curr] as Tsegment;
      thisTag = store.tags.find(
        (tag) => tag.id == thisEntity.id,
      );
      break;
    case "tags":
      selectedEntities = store.tags.filter(
        (tag) => tag.selected,
      ) as Array<Ttag>;
      thisEntity = selectedEntities[curr] as Ttag;
      thisTag = thisEntity;
      break;
    }
  }
}, [store.tab, curr]);

...

How can I clearly define the types of selectedEntities and thisEntity inside and outside the switch statement to resolve this TypeScript error?

Answer №1

When using TypeScript, the type of thisEntity is determined by its declaration (implicitly as any) rather than the assigned value (Tponto). This is because it is used in a callback function passed to find. TypeScript does not assume how find is implemented and considers that thisEntity could be reassigned to something other than a Tponto before or between function calls.

If you want to compare the id before calling find, you can read and store it beforehand.

switch (store.tab) {
  case 'points': { // lexical block for variable declaration
    ...
    thisEntity = selectedEntities[curr]
    const thisEntityId = thisEntity.id;
    thisTag = store.tags.find((tag) => tag.id == thisEntityId);
    break;
  }
  case 'segments':
    ...
}

It's recommended to keep all relevant variables local to each case, including the id, and utilize React.useState to manage component state updates when props like store and curr change.

If your goal is to minimize recalculations of these variables, consider using useMemo instead of useEffect, like this:

const { selectedEntities, thisEntity, thisTag } = useMemo(() => {
  let selectedEntities, thisEntity, thisTag;
  switch (store.tab) {
    case ... {
      selectedEntities = ...
      thisEntity = ...
      thisTag = ...
      break;
    }
    ...
  }
  // Types will likely be inferred as (Tpoint[]|Tsegment[]|Ttag[]) etc.
  return { selectedEntities, thisEntity, thisTag };
}, [store.tab, curr]);

// use variables here

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