distinctUntilKeyChanged

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

key	String key for object property lookup on each item.
compare	Optional. Default is undefined. Optional comparison function called to test if an item is distinct from the previous item in the source.

Returns

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

Description

If a comparator function is provided, then it will be called for each item to test for whether or not that value should be emitted.

If a comparator function is not provided, an equality check is used by default.

Examples

An example comparing the name of persons

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

 interface Person {
    age: number,
    name: string
 }

of(
    { 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(
    { 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