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

Examples

Example 1: distinctUntilChanged with basic values
// 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
// 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
// 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