combineLatest

signature: combineLatest(observables: ...Observable, project: function): Observable

When any observable emits a value, emit the latest value from each.

This operator can be used as either a static or instance method!

combineAll can be used to apply combineLatest to emitted observables when a source completes!

Why use combineLatest?

This operator is best used when you have multiple, long-lived observables that rely on eachother for some calculation or determination. Basic examples of this can be seen in example three, where events from multiple buttons are being combined to produce a count of each and an overall total, or a calculation of BMI from the RxJS documentation.

Be aware that combineLatest will not emit an initial value until each observable emits at least one value. This is the same behavior as withLatestFrom and can be a gotcha as there will be no output and no error but one (or more) of your inner observables is likely not functioning as intended, or a subscription is late.

Lastly, if you are working with observables that only emit one value, or you only require the last value of each before completion, forkJoin is likely a better option.

Examples

( example tests )

Example 1: Combining observables emitting at 3 intervals

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { timer, combineLatest } from 'rxjs';

//timerOne emits first value at 1s, then once every 4s
const timerOne = timer(1000, 4000);
//timerTwo emits first value at 2s, then once every 4s
const timerTwo = timer(2000, 4000);
//timerThree emits first value at 3s, then once every 4s
const timerThree = timer(3000, 4000);

//when one timer emits, emit the latest values from each timer as an array
const combined = combineLatest(timerOne, timerTwo, timerThree);

const subscribe = combined.subscribe(
  ([timerValOne, timerValTwo, timerValThree]) => {
    /*
      Example:
    timerOne first tick: 'Timer One Latest: 1, Timer Two Latest:0, Timer Three Latest: 0
    timerTwo first tick: 'Timer One Latest: 1, Timer Two Latest:1, Timer Three Latest: 0
    timerThree first tick: 'Timer One Latest: 1, Timer Two Latest:1, Timer Three Latest: 1
  */
    console.log(
      `Timer One Latest: ${timerValOne},
     Timer Two Latest: ${timerValTwo},
     Timer Three Latest: ${timerValThree}`
    );
  }
);

Example 2: combineLatest with projection function

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { timer, combineLatest } from 'rxjs';

//timerOne emits first value at 1s, then once every 4s
const timerOne = timer(1000, 4000);
//timerTwo emits first value at 2s, then once every 4s
const timerTwo = timer(2000, 4000);
//timerThree emits first value at 3s, then once every 4s
const timerThree = timer(3000, 4000);

//combineLatest also takes an optional projection function
const combinedProject = combineLatest(
  timerOne,
  timerTwo,
  timerThree,
  (one, two, three) => {
    return `Timer One (Proj) Latest: ${one}, 
              Timer Two (Proj) Latest: ${two}, 
              Timer Three (Proj) Latest: ${three}`;
  }
);
//log values
const subscribe = combinedProject.subscribe(latestValuesProject =>
  console.log(latestValuesProject)
);

Example 3: Combining events from 2 buttons

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { fromEvent, combineLatest } from 'rxjs';
import { mapTo, startWith, scan, tap, map } from 'rxjs/operators';

// helper function to set HTML
const setHtml = id => val => (document.getElementById(id).innerHTML = val);

const addOneClick$ = id =>
  fromEvent(document.getElementById(id), 'click').pipe(
    // map every click to 1
    mapTo(1),
    startWith(0),
    // keep a running total
    scan((acc, curr) => acc + curr),
    // set HTML for appropriate element
    tap(setHtml(`${id}Total`))
  );

const combineTotal$ = combineLatest(addOneClick$('red'), addOneClick$('black'))
  .pipe(map(([val1, val2]) => val1 + val2))
  .subscribe(setHtml('total'));

HTML

<div>
  <button id='red'>Red</button>
  <button id='black'>Black</button>
</div>
<div>Red: <span id="redTotal"></span> </div>
<div>Black: <span id="blackTotal"></span> </div>
<div>Total: <span id="total"></span> </div>

Additional Resources

• combineLatest - Official docs
• Combining streams with combineLatest - John Linquist
• Combination operator: combineLatest - André Staltz

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