Let's go through each operator and explain them along with examples. We'll also discuss what happens if any of the requests fail when using `switchMap`, `mergeMap`, or `forkJoin`.

1. switchMap: This operator is used to map each source value to an inner observable, and it only emits the values from the most recent inner observable. If a new source value arrives before the previous inner observable completes, it will switch to the new inner observable and unsubscribe from the previous one. Example:

import { of, interval } from 'rxjs';
import { switchMap } from 'rxjs/operators';
  const $switchMap = from([1,2,3,4]).pipe(switchMap(data=>{
    return of(data).pipe(delay(500))
  }));

  $switchMap.subscribe(data=>{
    console.log('switch map data', data); // 4 as switchMap cancels all previous observables when new observable is emitted
  })

In this example, `sourceObservable` emits values every second. For each value emitted, `switchMap` creates an inner observable using `from` operator and emits the value from the source observable after a delay of one second. If a new value is emitted before the previous inner observable completes, it switches to the new inner observable and cancels the previous one. Therefore, only the most recent inner observable value will be emitted. 2. mergeMap: This operator maps each source value to an inner observable and merges the values from multiple inner observables into a single observable. It does not cancel or unsubscribe from any inner observables. Example:

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

// Create an observable that emits three values
const sourceObservable = of(1, 2, 3);

// Use mergeMap to merge the values from the inner observables
const resultObservable = sourceObservable.pipe(
  mergeMap((value) => {
    // Create an inner observable that emits the value after a delay
    return of(value).pipe(delay(1000));
  })
);

// Subscribe to the result observable
resultObservable.subscribe((value) => {
  console.log(value); // 1 2 3 
});

In this example, `sourceObservable` emits three values: 1, 2, and 3. For each value emitted, `mergeMap` creates an inner observable using `of` operator and emits the value after a delay of one second. Since `mergeMap` does not cancel or switch between inner observables, all the values from each inner observable will be merged into a single observable and emitted in the order they complete. 3. forkJoin: This operator takes an array of observables and waits for all the source observables to complete. Once they all complete, it emits an array of the last emitted values from each observable. Example:

import { forkJoin, of, throwError } from 'rxjs';

 const observables = [
     of(1,2,3).pipe(delay(500)),
    from([10,11,12])
  ]

  const $forkJoin = forkJoin(observables);

  $forkJoin.subscribe(data=>{
    console.log('forkjoin data', data); // [3,12] as forkJoin will return last emitted values of each observable
  })

In this example, `forkJoin` takes an array of observables, including two observables that emit 'A' and 'B' respectively after a delay, and an observable that throws an error after a delay. `forkJoin` waits for all the observables to complete, and once they complete, it emits an array of the last emitted values from each observable. However, if any of the observables in `forkJoin` throws an error, the error will be propagated to the error callback of the `subscribe` method. 4. combineLatest: This operator combines the latest values from multiple observables into a single observable. It emits an array of the latest values whenever any of the source observables emit a new value. Example:

import { combineLatest, interval } from 'rxjs';
  const observables = [
     of(1,2,3,4),
     from([10,11,12])
  ]

  const $combineLatest = combineLatest(observables);

  $combineLatest.subscribe(data=> {
    console.log('combineLatest data', data);
  })
/*  output */
// [4, 10]
// [4,11]
// [4,12]

If our observable is as shown below :

  const observables = [
    of(1,2,3,4).pipe(delay(500)), 
    from([10,11,12])
  ]

then output will be as follows

// [12,1]
// [12,2]
// [12,3]
// [12,4]

The console.log() output from the project function shows that the last emitted value from the first completed observable is used in all calculations. It is combined with each of the second observable values .Hence: if one observable emits values before the others do, then those values are lost. 5. concatMap: This operator maps each source value to an inner observable and concatenates the values from each inner observable sequentially. It waits for each inner observable to complete before moving on to the next one. Use concatMap over mergeMap when order is important for you. Example:

import { of } from 'rxjs';
import { concatMap, delay } from 'rxjs/operators';

// Create an observable that emits three values
const sourceObservable = of(1, 2, 3);

// Use concatMap to concatenate the values from the inner observables
const resultObservable = sourceObservable.pipe(
  concatMap((value) => {
    // Create an inner observable that emits the value after a delay
    return of(value).pipe(delay(1000));
  })
);

// Subscribe to the result observable
resultObservable.subscribe((value) => {
  console.log(value); // 1  2  3
});

In this example, `sourceObservable` emits three values: 1, 2, and 3. For each value emitted, `concatMap` creates an inner observable using `of` operator and emits the value after a delay of one second. It waits for each inner observable to complete before moving on to the next one. Therefore, the values from each inner observable will be emitted sequentially in the order they were mapped. 6. exhaustMap: The exhaustMap operator works by mapping each source value to an observable, and then subscribing to that observable. It ensures that only one inner observable is active at a time. If a new source value arrives while an inner observable is still active, the new value is ignored until the inner observable completes. Here's an example to demonstrate the usage of exhaustMap in Angular:

 const $exhaustMap = from([1,2,3,4]).pipe(
    exhaustMap(data=>{
      return of(data).pipe(delay(500)); 
    })
  )

  $exhaustMap.subscribe(data=> {
    console.log('exhaustMap data', data); //1
  })

Regarding what happens if any of the requests fail in `switchMap`, `mergeMap`, or `forkJoin`: - switchMap: If any of the inner observables created by `switchMap` throws an error, the error will be propagated to the error callback of the `subscribe` method. Additionally, the subscription to the previous inner observable will be canceled, and `switchMap` will switch to the new inner observable. - mergeMap: If any of the inner observables created by `mergeMap` throws an error, the error will be propagated to the error callback of the `subscribe` method. However, the error in one inner observable will not affect the other inner observables. `mergeMap` will continue to merge values from other inner observables. -forkJoin: If any of the observables passed to `forkJoin` throws an error, the error will be propagated to the error callback of the `subscribe` method. In this case, `forkJoin` will not emit any result value. If you need to handle individual errors of each observable in `forkJoin`, you can use the `catchError` operator within each observable before passing them to `forkJoin`. It's important to note that error handling strategies and behavior may vary based on your specific use case and how you handle errors within your code.