在Rxjs中使用mergeMap时输出未定义

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

const source$ = of(1, 2, 3);

source$.pipe(
  mergeMap(value => {
    if (value === 2) {
      return undefined; // 模拟未定义的情况
    }
    return of(value * 2);
  })
).subscribe(result => {
  console.log(result);
});

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

const source$ = of(1, 2, 3);

source$.pipe(
  flatMap(value => {
    if (value === 2) {
      return undefined; // 模拟未定义的情况
    }
    return of(value * 2);
  })
).subscribe(result => {
  console.log(result);
});