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	K	String key for object property lookup on each item.
compare	(x: T[K], y: T[K]) => boolean	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, distinctUntilKeyChanged } from 'rxjs';

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, distinctUntilKeyChanged } from 'rxjs';

of(
  { age: 4, name: 'Foo1' },
  { age: 7, name: 'Bar' },
  { age: 5, name: 'Foo2' },
  { age: 6, name: 'Foo3' }
).pipe(
  distinctUntilKeyChanged('name', (x, y) => 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