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distinctUntilSomeChanged

RxJS operator that emits items from the source Observable that are distinct by comparison from the previous item, comparing each key in the keys array. It is named distinctUntilSomeChanged because it iterates the keys and uses Array.prototype.some to decide whether values are distinct.

Pass a KeyCompareMap to fine-grain the distinct check for specific keys.

Why distinct-checking matters: see Reactive state: global vs local, RxState + signals.

Signature​

function distinctUntilSomeChanged<T extends object, K extends keyof T>(keys: K[], keyCompareMap?: KeyCompareMap<T>): MonoTypeOperatorFunction<T>;

Parameters​

ParameterTypeMeaning
keysK[]String keys for object-property lookup on each item.
keyCompareMapKeyCompareMap<T>Optional. Custom compare functions for specific keys; keys without an entry fall back to strict equality (===).

Returns​

MonoTypeOperatorFunction<T>. The source type is preserved; only the emission cadence changes.

Import​

import { distinctUntilSomeChanged } from '@rx-angular/state/selections';

Example​

import { of } from 'rxjs';
import { distinctUntilSomeChanged, KeyCompareMap } from '@rx-angular/state/selections';

interface Person {
  age: number;
  name: string;
}

const customComparison: KeyCompareMap<Person> = {
  name: (oldName, newName) => oldName.substring(0, 2) === newName.substring(0, 2),
};

of({ age: 4, name: 'Hans' }, { age: 7, name: 'Sophie' }, { age: 5, name: 'Han Solo' }, { age: 5, name: 'HanSophie' })
  .pipe(distinctUntilSomeChanged(['age', 'name'], customComparison))
  .subscribe((x) => console.log(x));

// displays:
// { age: 4, name: 'Hans' }
// { age: 7, name: 'Sophie' }
// { age: 5, name: 'Han Solo' }

Signal alternative​

Signal graphs are already distinct-by-value: a computed() recomputes only when a read signal changes, so a component reading state.signal(...) / computed(...) does not need this operator. Keep distinctUntilSomeChanged for RxJS pipelines where you are gating an Observable on a subset of keys before it reaches state.

See also​