Rxjs加减号按钮，最小最大值不为负值

import { BehaviorSubject } from 'rxjs';

const valueSubject = new BehaviorSubject(0);

import { fromEvent } from 'rxjs';

const addButton = document.getElementById('add-button');
const subtractButton = document.getElementById('subtract-button');

const addClick$ = fromEvent(addButton, 'click');
const subtractClick$ = fromEvent(subtractButton, 'click');

import { merge } from 'rxjs';

const click$ = merge(addClick$, subtractClick$);

import { scan } from 'rxjs/operators';

const newValue$ = click$.pipe(
  scan((acc, curr) => {
    if (curr.target.id === 'add-button') {
      return acc + 1;
    } else {
      return Math.max(0, acc - 1);
    }
  }, 0)
);

newValue$.subscribe(value => {
  valueSubject.next(value);
});

const valueElement = document.getElementById('value');

valueSubject.subscribe(value => {
  valueElement.textContent = value.toString();
});