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>: 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<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' }