narrowing filter example

compiler-output.txt

[23:21:45] File change detected. Starting incremental compilation...

src/models/District.ts:73:7 - error TS2322: Type '(item: General) => boolean' is not assignable to type '(item: General) => item is Specific1'.
  Signature '(item: General): boolean' must be a type predicate.

73 const predicate3:(item:General) => item is Specific1 = item => item.kind=="Specific1";
         ~~~~~~~~~~

src/models/District.ts:76:65 - error TS2345: Argument of type '(item: General) => boolean' is not assignable to parameter of type 'TypePredicate<General, General>'.
  Signature '(item: General): boolean' must be a type predicate.

76 const listOfOnlySubtypes2 = narrowingFilterFunction(unfiltered, predicate2)
                                                                   ~~~~~~~~~~

[23:21:46] Found 2 errors. Watching for file changes.

narrowing-filter-example.ts

export type TypePredicate<U extends T, T> = (item:T) => item is U;
export type NarrowingFilterSignature = <U extends T, T>(items:T[], predicate:TypePredicate<U, T>) => U[];
export type General = Specific1 | Specific2;
export interface Specific1 {
    kind: "Specific1",
    specific1OnlyMethod():void,
}

export interface Specific2 {
  kind: "Specific2",
  specific2OnlyMethod():void,
}

declare const unfiltered:General[];
declare const narrowingFilterFunction:NarrowingFilterSignature;
  
function predicate1(item:General): item is Specific1 {
  return item.kind=="Specific1";
}

function predicate2(item:General) {
  return item.kind=="Specific1";
}

const predicate3:(item:General) => item is Specific1 = item => item.kind=="Specific1";

const listOfOnlySubtypes1 = narrowingFilterFunction(unfiltered, predicate1)
const listOfOnlySubtypes2 = narrowingFilterFunction(unfiltered, predicate2)
const listOfOnlySubtypes3 = narrowingFilterFunction(unfiltered, predicate3)

listOfOnlySubtypes1[0].specific1OnlyMethod();
listOfOnlySubtypes2[0].specific1OnlyMethod();
listOfOnlySubtypes3[0].specific1OnlyMethod();