distinctUntilChanged

Ignores the same value repeated back-to-back.

Signature

distinctUntilChanged(comparator?: (a: T, b: T) => boolean, keySelector?: (v: T) => K): MonoTypeOperatorFunction

Marble diagram

in:     1-1-2-2-3-1-|
result: 1---2---3-1-|

What it does

Remembers the last emitted value and compares each new one to it (=== by default), passing it on only if it differs. An optional keySelector compares by a chosen object field only.

When to use

Typeahead search (the input matched the previous one — no request needed). Shielding subscribers from redundant updates. Cutting re-renders in Angular.

Gotcha

It compares only with the previous value, not the whole history. For global uniqueness, use the distinct operator.

distinctUntilChanged