I have type Item: type Item = 'itemA' | 'itemB' | 'itemC' | 'itemD';

I have 2 types of items – QuantityBasedItem and BooleanBasedItem:

I have a function that get an item and if the item type is QuantityBasedItem, it also get the quantity:

type MyType<T extends Item> = {
  name: T;
  requirements: T extends QuantityBasedItem ? { units: number } : undefined;
};

function test<T extends Item>(itemData: MyType<T>){
    getItemData(itemData)
}

First issue:

test({name: 'itemB', requirements: undefined}); // works OK
test({name: 'itemB'}); // ERROR. i don't want to have to send unnecessary data (undefined) `test({item: 'itemB', requirements: undefined})`

Second issue:

const getIsQuantityBasedItem = (x: Item): x is QuantityBasedItem => x === 'itemA';

export const getItemData = <T extends Item>(
 itemData: MyType<T>,
) => {
  if(getIsQuantityBasedItem(itemData.name)){ 
    // ISSUE 2: ERROR: the type is not inferred. the item is quantity based => item.requirements is exist and has units inside it, but typescript does't infer it
    return getQuantityBasedItemData(itemData.name, itemData.requirements.units)
  } else {
    return getBooleanBasedItemData(itemData.name)
  }

};


const getBooleanBasedItemData = (item: BooleanBasedItem) => (item);
const getQuantityBasedItemData = (item: QuantityBasedItem, units: number) => ({item, units});

playground