RxJS distinctUntilChanged - object comparison

Question

I have a stream of objects and I need to compare if the current object is not the same as the previous and in this case emit a new value. I found distinctUntilChanged operator should do exactly what I want, but for some reason, it never emits value except the first one. If I remove distinctUntilChanged values are emitted normally.

My code:

export class SettingsPage {
    static get parameters() {
        return [[NavController], [UserProvider]];
    }

    constructor(nav, user) {
        this.nav = nav;
        this._user = user;

        this.typeChangeStream = new Subject();
        this.notifications = {};
    }

    ngOnInit() {

        this.typeChangeStream
            .map(x => {console.log('value on way to distinct', x); return x;})
            .distinctUntilChanged(x => JSON.stringify(x))
            .subscribe(settings => {
                console.log('typeChangeStream', settings);
                this._user.setNotificationSettings(settings);
            });
    }

    toggleType() {
        this.typeChangeStream.next({
            "sound": true,
            "vibrate": false,
            "badge": false,
            "types": {
                "newDeals": true,
                "nearDeals": true,
                "tematicDeals": false,
                "infoWarnings": false,
                "expireDeals": true
            }
        });
    }

    emitDifferent() {
        this.typeChangeStream.next({
            "sound": false,
            "vibrate": false,
            "badge": false,
            "types": {
                "newDeals": false,
                "nearDeals": false,
                "tematicDeals": false,
                "infoWarnings": false,
                "expireDeals": false
            }
        });
    }
}

Answer 1

I had the same problem, and fixed it with using JSON.stringify to compare the objects:

.distinctUntilChanged((a, b) => JSON.stringify(a) === JSON.stringify(b))

Dirty but working code.

Answer 2

I finally figure out where problem is. Problem was in version of RxJS, in V4 and earlier is different parameters order than V5.

RxJS 4:

distinctUntilChanged = function (keyFn, comparer)

RxJS 5:

distinctUntilChanged = function (comparer, keyFn)

In every docs today, you can find V4 parameters order, beware of that!

Answer 3

When you have lodash in your application anyway, you can simply utilize lodash's isEqual() function, which does a deep comparison and perfectly matches the signature of distinctUntilChanged():

.distinctUntilChanged(isEqual),

Or if you have _ available (which is not recommended anymore these days):

.distinctUntilChanged(_.isEqual),

Answer 4

You can also wrap the original distinctUntilChanged function.

function distinctUntilChangedObj<T>() {
  return distinctUntilChanged<T>((a, b) => JSON.stringify(a) === JSON.stringify(b));
}

This lets you use it just like the original.

$myObservable.pipe(
  distinctUntilChangedObj()
)

Answer 5

From RxJS v6+ there is distinctUntilKeyChanged

https://www.learnrxjs.io/operators/filtering/distinctuntilkeychanged.html

const source$ = from([
  { name: 'Brian' },
  { name: 'Joe' },
  { name: 'Joe' },
  { name: 'Sue' }
]);

source$
  // custom compare based on name property
  .pipe(distinctUntilKeyChanged('name'))
  // output: { name: 'Brian }, { name: 'Joe' }, { name: 'Sue' }
  .subscribe(console.log);

Answer 6

Mehdi's solution although fast but wouldn't work if the order is not maintained. Using one of deep-equal or fast-deep-equal libraries:

.distinctUntilChanged((a, b) => deepEqual(a, b))

Answer 7

The answers suggesting a deep compare all fail if the observable is actually emitting a modified version of the same object, as the "last" value that is passed to the comparison will be the same as the current, every time.

We've hit this using a BehaviorSubject where it just so happens that the same object is passed to .next() each time.

In this case, there is no solution using the default distinctUntilChanged, no matter what comparison function you use.

Answer 8

You can also make your own filter:

function compareObjects(a: any, b: any): boolean {
  return JSON.stringify(a) === JSON.stringify(b);
}

export function distinctUntilChangedObject() {
  return function<T>(source: Observable<T>): Observable<T> {
    return new Observable(subscriber => {
      let prev: any;
      let first = true;
      source.subscribe({
        next(value) {
          if (first) {
            prev = value;
            subscriber.next(value);
            first = false;
          } else if (!compareObjects(prev, value)) {
            prev = value;
            subscriber.next(value);
          }
        },
        error(error) {
          subscriber.error(error);
        },
        complete() {
          subscriber.complete();
        }
      });
    });
  };
}

Answer 9

If you change the value mutably, none of the other answers work. If you need to work with mutable data structures, you can use distinctUntilChangedImmutable, which deep copies the previous value and if no comparison function is passed in, will assert that the previous and the current values deep equals each other (not === assertion).