distinctUntilChanged
signature: distinctUntilChanged(compare: function): Observable
distinctUntilChanged(compare: function): Observable
Only emit when the current value is different than the last.
:bulb: distinctUntilChanged uses ===
comparison by default, object references must match!
:bulb: If you want to compare based on an object property, you can use distinctUntilKeyChanged
instead!
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);
Related Recipes
[Save Indicator]('../../recipes/save-indicator.md)
Additional Resources
:newspaper: - Official docs
Filtering operator: distinct and distinctUntilChanged
:video_camera: :dollar: - André Staltz
:file_folder: Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/distinctUntilChanged.ts
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