​​​​​​​Rxjs map, mergeMap 和 switchMap 的区别与联系

import { of } from 'rxjs';
import { map } from 'rxjs/operators';
const observable = of([
    {
        name: "Parwinder",
        character: "Calcifer"
    },
    {
        name: "Laure",
        character: "Alchemist"
    },
    {
        name: "Eliu",
        character: "X-Men"
    },
    {
        name: "Robert",
        character: "Link"
    }
]);
observable.pipe(
    map(arr => arr.map(person => person.character)) // loops over objects and returns characters
).subscribe(
    char => console.log(char) // ["Calcifer", "Alchemist", "X-Men", "Link"]
);

import { of, from } from 'rxjs';
import { map } from 'rxjs/operators';
const dummyApi = (character) => { // fake api call function
  return of(`API response for character: ${character}`).pipe(
    delay(1000) // the fake api takes 1 second
  );
}
from(["Calcifer", "Alchemist", "X-Men", "Link"]) // characters I need to get information for
.pipe(
  map(arr => dummyApi(arr)) // generates 4 new Observables
).subscribe( // subscribing Observable (outer) of 4 Observables (inner)
  data => data.subscribe(i => console.log(i)) // subscribing to inner Observables
)

import { of, from } from 'rxjs';
import { mergeMap } from 'rxjs/operators';
const dummyApi = (character) => {
  return of(`API response for character: ${character}`)..pipe(
    delay(1000)
  );
}
from(["Calcifer", "Alchemist", "X-Men", "Link"])
.pipe(
  mergeMap(arr => dummyApi(arr)) // gets 4 Observable as API response and merges them
).subscribe( // we subscribe to one mapped and merged Observable
  data => console.log(data)
)

import { of, from } from 'rxjs';
import { switchMap, delay } from 'rxjs/operators';
const dummyApi = (character) => {
  return of(`API response for character: ${character}`).pipe(
    delay(1000)
  );
}
from(["Calcifer", "Alchemist", "X-Men", "Link"])
.pipe(
  switchMap(arr => dummyApi(arr))
).subscribe(
  data => console.log(data) // API response for character: Link
)

import { of, from } from 'rxjs';
import { switchMap, delay } from 'rxjs/operators';
const dummyApi = (character) => {
  return of(`Search result for keyword: ${character}`).pipe(
    delay(1000)
  );
}
from(["C", "Ch", "Cha", "Chas", "Chase"]) // mimic key input in text field
.pipe(
  switchMap(arr => dummyApi(arr))
).subscribe(
  data => console.log(data) // Search result for keyword: Chase
)