scan

scan<T, R>(
  accumulator: (acc: R, value: T, index: number) => R,
  seed?: T | R
): OperatorFunction<T, R>

import { of } from 'rxjs';
import { scan } from 'rxjs/operators';

const source = of(1, 2, 3);
const example = source.pipe(scan((acc, curr) => acc + curr, 0));
// 输出累加值
example.subscribe(val => console.log(val));
// 输出:
// 1,3,6

import { Subject } from 'rxjs';
import { scan } from 'rxjs/operators';

const subject = new Subject();
// scan 示例，随着时间的推移构建对象
const example = subject.pipe(scan((acc, curr) => Object.assign({}, acc, curr), {}));

// 输出累加值
example.subscribe(val => console.log('Accumulated object:', val));
// subject 投射的值会被添加成对象的属性
// {name: 'Joe'}
subject.next({ name: 'Joe' });
// {name: 'Joe', age: 30}
subject.next({ age: 30 });
// {name: 'Joe', age: 30, favoriteLanguage: 'JavaScript'}
subject.next({ favoriteLanguage: 'JavaScript' });

import { interval } from 'rxjs';
import { scan, map, distinctUntilChanged } from 'rxjs/operators';

// 累加数组中的值，并随机投射此数组中的值
const scanObs = interval(1000)
  .pipe(
    scan((a, c) => [...a, c], []),
    map(r => r[Math.floor(Math.random() * r.length)]),
    distinctUntilChanged()
  )
  .subscribe(console.log);