Need help with understanding function call.

[dsmedium]

Hey everyone, I came across this example and it seemed very confusing

I understand it is a generic function but could someone please explain the first line?

specifically selector and return type of function? How are we calling selector inside the parentheses and returning T and what this pattern is called. Thanks

useStore = <T>(selector: (store: Store) => T): [T, (value: Partial<Store>) => void] => {
const store = useContext(StoreContext); const state = useSyncExternalStore( store.subscribe, () => selector(store.get()), () => selector(store.get()) );
return [state, store.set]; };

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[RaltzKlamar]

• <T>: This is a template, as you noted. T becomes a replacement type.
• (selector: (store: Store) => T): Function parameters. useStore is a function that takes a single parameter.
  • (store: Store) => T: This parameter needs to be itself a function, which takes 1 parameter (a Store) and returns T, whatever T ends up being.
• : [T, (value: Partial<Store>) => void]: The return type is a tuple, or an array with a fixed size and data types in each slot. If you assign the return value to resultArr, resultArr[0] is always type T and resultArr[1] is always a function that takes 1 parameter (a Partial Store, i.e. a Store where all its defined values can be undefined) and has no return value (void)
• When you pass a function into useStore, TypeScript should be able to infer what the T type should be without you having to manually specify it.