distinctUntilChanged

signature: distinctUntilChanged(compare: function): Observable

Only emit when the current value is different than the last.

distinctUntilChanged uses === comparison by default, object references must match!

Examples

Example 1: distinctUntilChanged with basic values

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';

//only output distinct values, based on the last emitted value
const myArrayWithDuplicatesInARow = from([1, 1, 2, 2, 3, 1, 2, 3]);

const distinctSub = myArrayWithDuplicatesInARow
  .pipe(distinctUntilChanged())
  //output: 1,2,3,1,2,3
  .subscribe(val => console.log('DISTINCT SUB:', val));

const nonDistinctSub = myArrayWithDuplicatesInARow
  //output: 1,1,2,2,3,1,2,3
  .subscribe(val => console.log('NON DISTINCT SUB:', val));

Example 2: distinctUntilChanged with objects

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';

const sampleObject = { name: 'Test' };
//Objects must be same reference
const myArrayWithDuplicateObjects = from([
  sampleObject,
  sampleObject,
  sampleObject
]);
//only out distinct objects, based on last emitted value
const nonDistinctObjects = myArrayWithDuplicateObjects
  .pipe(distinctUntilChanged())
  //output: 'DISTINCT OBJECTS: {name: 'Test'}
  .subscribe(val => console.log('DISTINCT OBJECTS:', val));

Additional Resources

• distinctUntilChanged - Official docs
• Filtering operator: distinct and distinctUntilChanged - André Staltz

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