distinctUntilChanged

signature: distinctUntilChanged(compare: function): Observable

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

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

πŸ’‘ If you want to compare based on an object property, you can use distinctUntilKeyChanged instead!

​​Ultimate RxJS​​

Examples

Example 1: distinctUntilChanged with basic values

( StackBlitz )

// RxJS v6+
import { from } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';
​
// only output distinct values, based on the last emitted value
const source$ = from([1, 1, 2, 2, 3, 3]);
​
source$
.pipe(distinctUntilChanged())
// output: 1,2,3
.subscribe(console.log);

Example 2: distinctUntilChanged with objects

( StackBlitz )

// RxJS v6+
import { from } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';
​
const sampleObject = { name: 'Test' };
​
//Objects must be same reference
const source$ = from([sampleObject, sampleObject, sampleObject]);
​
// only emit distinct objects, based on last emitted value
source$
.pipe(distinctUntilChanged())
// output: {name: 'Test'}
.subscribe(console.log);

Example 3: Using custom comparer function

( StackBlitz )

// RxJS v6+
import { from } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';
​
// only output distinct values, based on the last emitted value
const source$ = from([
{ name: 'Brian' },
{ name: 'Joe' },
{ name: 'Joe' },
{ name: 'Sue' }
]);
​
source$
// custom compare for name
.pipe(distinctUntilChanged((prev, curr) => prev.name === curr.name))
// output: { name: 'Brian }, { name: 'Joe' }, { name: 'Sue' }
.subscribe(console.log);

Additional Resources

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