What are key RxJS operators and the flattening operators?

Question

Accepted Answer

**RxJS operators** transform, filter, and combine Observable streams. The most important — and most confused — are the four **flattening operators** that handle Observables-of-Observables (e.g. an HTTP call triggered by each emission of another stream).

## Common transformation/filter operators

```ts
import { map, filter, tap, debounceTime, distinctUntilChanged, catchError } from "rxjs/operators";

source$.pipe(
  map(x => x * 2),                 // transform each value
  filter(x => x > 10),             // keep values passing a test
  tap(x => console.log(x)),        // side effect, doesn't change the stream
  debounceTime(300),               // wait for a pause before emitting (search inputs)
  distinctUntilChanged(),          // ignore consecutive duplicates
);
```

## The four flattening operators (the tricky part)

When each value from a source needs to trigger **another** Observable (like an HTTP request), you must "flatten" the inner Observable. The four differ in **how they handle overlap**:

```text
switchMap   → cancels the previous inner Observable when a new value arrives
mergeMap    → runs all inner Observables concurrently (no cancellation)
concatMap   → queues them, runs one at a time in order
exhaustMap  → ignores new values while an inner Observable is still running
```

## When to use each

```ts
// switchMap — type-ahead search: cancel the stale request when the user types again
this.searchInput.valueChanges.pipe(
  debounceTime(300),
  switchMap(query => this.api.search(query)) // only the LATEST query's result matters
).subscribe(results => this.results = results);

// mergeMap — fire independent requests in parallel (order doesn't matter)
from(ids).pipe(mergeMap(id => this.api.get(id)));

// concatMap — operations that MUST run in order (e.g. sequential saves)
actions$.pipe(concatMap(action => this.api.save(action)));

// exhaustMap — ignore repeated clicks while a login is in flight
this.loginClick$.pipe(exhaustMap(() => this.auth.login())); // ignore spam clicks
```

## A mnemonic

```text
switchMap  = "switch to the new one" (cancel old) → search
mergeMap   = "merge all" (parallel)              → independent requests
concatMap  = "concatenate" (sequential)          → ordered operations
exhaustMap = "exhaust current first" (ignore new) → prevent double-submit
```

## Why it matters

The flattening operators are among the most important — and most commonly misused — RxJS tools in Angular.

Choosing wrong causes real bugs: using `mergeMap` for search shows stale results (no cancellation); using `switchMap` for sequential saves cancels them mid-flight.

Knowing that `switchMap` cancels (ideal for search), `concatMap` orders, `mergeMap` parallelizes, and `exhaustMap` blocks duplicates is essential for correct async handling in any RxJS-heavy Angular code.