查看当前项目安装的包的版本：
npm list --depth=0
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import { Observable, from, of, interval } from ‘rxjs‘;
import { filter, map, scan, take } from ‘rxjs/operators‘;
const source = interval(1000)
source.pipe(filter(val => val%2 ==0), scan((x,y)=>x+y), take(4)).subscribe(val => console.log(val));
执行结果如下：
原因分析：
https://www.jianshu.com/p/d8cb71554008
原文：https://www.cnblogs.com/oxspirt/p/11813204.html