withLatestFrom

signature: withLatestFrom(other: Observable, project: Function): Observable

Also provide the last value from another observable.

💡 If you want the last emission any time a variable number of observables emits, try combinelatest!

Why use withLatestFrom?

The withLatestFrom operator is your best friend when you have one main observable whose emissions depend on the latest values from one or more other observables. Think of it as a one-way data flow, where the primary observable takes the lead and other observables chime in with their most recent values.

A classic example to remember withLatestFrom is a chat application that needs to send a message with a user's current location. The message sending event (main observable) combines with the latest location data (another observable) to form the final message object.

Keep in mind that withLatestFrom only emits a value when the main observable emits, and after each additional observable has emitted at least once. This can catch you off guard, as you might not see any output or errors while one of the observables isn't behaving as expected, or a subscription is delayed.

If you need to combine values from multiple observables that emit more than once and are interdependent, consider using combineLatest instead. And for scenarios where observables emit only once or you just need their last values, forkJoin might be a more suitable choice.

Examples

Example 1: Latest value from quicker second source

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs';

//emit every 5s
const source = interval(5000);
//emit every 1s
const secondSource = interval(1000);
const example = source.pipe(
  withLatestFrom(secondSource),
  map(([first, second]) => {
    return `First Source (5s): ${first} Second Source (1s): ${second}`;
  })
);
/*
  "First Source (5s): 0 Second Source (1s): 4"
  "First Source (5s): 1 Second Source (1s): 9"
  "First Source (5s): 2 Second Source (1s): 14"
  ...
*/
const subscribe = example.subscribe(val => console.log(val));

Example 2: Slower second source

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs';

//emit every 5s
const source = interval(5000);
//emit every 1s
const secondSource = interval(1000);
//withLatestFrom slower than source
const example = secondSource.pipe(
  //both sources must emit at least 1 value (5s) before emitting
  withLatestFrom(source),
  map(([first, second]) => {
    return `Source (1s): ${first} Latest From (5s): ${second}`;
  })
);
/*
  "Source (1s): 4 Latest From (5s): 0"
  "Source (1s): 5 Latest From (5s): 0"
  "Source (1s): 6 Latest From (5s): 0"
  ...
*/
const subscribe = example.subscribe(val => console.log(val));

Related Recipes

Additional Resources

📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/withLatestFrom.ts

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