ngrx/store reselect

reselect.ts

import { defaultMemoize as memoize } from 'reselect';

export interface StreamSelector<A,B> {
  (input$: Observable<T>): Observable<R>;
}

export interface Share {
  <A,B>(): StreamSelector<A,B>;
  <A,B,C>(c: C): StreamSelector<A,B>;
  <A,B,C,D>(c: C, d: D): StreamSelector<A,B>;
  <A,B,C,D,E>(c: C, d: D, e: E): StreamSelector<A,B>;
  <A,B,C,D,E,F>(c: C, d: D, e: E, f: F): StreamSelector<A,B>;
  <A,B,C,D,E,F,G>(c: C, d: D, e: E, f: F, g: G): StreamSelector<A,B>;
  <A,B,C,D,E,F,G,H>(c: C, d: D, e: E, f: F, g: G, h: H): StreamSelector<A,B>;
  <A,B,C,D,E,F,G,H,I>(c: C, d: D, e: E, f: F, g: G, h: H, i: I): StreamSelector<A,B>;
  <A,B,C,D,E,F,G,H,I,J>(c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J): StreamSelector<A,B>;
}

export const share: Share = function(fn) {
  let last, memoized;
  
  const factory = memoize(source);
  
  return function (...args) {
    let result = factory(...args);
    
    if (result !== last) {
      last = result;
      memoized = memoize((source) => result(source).share());
    }
    
    return memoized;
  }
}

How does this work?