Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from the previous item, using a property accessed by using the key provided to check if the two items are distinct.

distinctUntilKeyChanged<T, K extends keyof T>(key: K, compare?: (x: T[K], y: T[K]) => boolean): MonoTypeOperatorFunction<T>

Parameters

Returns

MonoTypeOperatorFunction<T>: An Observable that emits items from the source Observable with distinct values based on the key specified.

Description

It's like distinctUntilChanged, but the distinct comparison uses a key to access a property.

distinctUntilKeyChanged emits all items of the source Observable, wich are distinct by comparison. The comparison checks if the previous item is distinct from the current item, using a key to access a property. If a comparator function is provided, then it will be called for each item with the property key to test for whether or not that value should be emitted.

Examples

An example comparing the name of persons

import { of } from 'rxjs';
import { distinctUntilKeyChanged } from 'rxjs/operators';

 interface Person {
    age: number,
    name: string
 }

of<Person>(
    { age: 4, name: 'Foo'},
    { age: 7, name: 'Bar'},
    { age: 5, name: 'Foo'},
    { age: 6, name: 'Foo'},
  ).pipe(
    distinctUntilKeyChanged('name'),
  )
  .subscribe(x => console.log(x));

// displays:
// { age: 4, name: 'Foo' }
// { age: 7, name: 'Bar' }
// { age: 5, name: 'Foo' }

An example comparing the first letters of the name

import { of } from 'rxjs';
import { distinctUntilKeyChanged } from 'rxjs/operators';

interface Person {
    age: number,
    name: string
 }

of<Person>(
    { age: 4, name: 'Foo1'},
    { age: 7, name: 'Bar'},
    { age: 5, name: 'Foo2'},
    { age: 6, name: 'Foo3'},
  ).pipe(
    distinctUntilKeyChanged('name', (x: string, y: string) => x.substring(0, 3) === y.substring(0, 3)),
  )
  .subscribe(x => console.log(x));

// displays:
// { age: 4, name: 'Foo1' }
// { age: 7, name: 'Bar' }
// { age: 5, name: 'Foo2' }

See Also

• distinct
• distinctUntilChanged