combinelatest.md

combineLatest

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

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

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💡 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!

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Examples

( example tests )

Example 1: Combining observables emitting at 3 intervals

( jsBin | jsFiddle )

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

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

const subscribe = combined.subscribe(latestValues => {
	//grab latest emitted values for timers one, two, and three
	const [timerValOne, timerValTwo, timerValThree] = latestValues;
  /*
  	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

( jsBin | jsFiddle )

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

//combineLatest also takes an optional projection function
const combinedProject = Rx.Observable
.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

( jsBin | jsFiddle )

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

const addOneClick$ = id => Rx.Observable
    .fromEvent(document.getElementById(id), 'click')
    // map every click to 1
    .mapTo(1)
    .startWith(0)
    // keep a running total
    .scan((acc, curr) => acc + curr)
    // set HTML for appropriate element
    .do(setHtml(`${id}Total`))
  
  
const combineTotal$ = Rx.Observable
  .combineLatest(
    addOneClick$('red'),
    addOneClick$('black')
  )
  .map(([val1, val2]) => val1 + val2)
  .subscribe(setHtml('total'));

HTML

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

Additional Resources

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

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/operator/combineLatest.ts