18. distinctUntilChanged: drop the repeats

distinctUntilChanged — no neighboring duplicates

Sometimes a stream emits the same value twice in a row. A user presses and releases a key, say, but the text didn't change. Firing a fresh request for an identical value is wasteful. distinctUntilChanged() strips those repeats out.

['a', 'a', 'b', 'b', 'a'] → distinctUntilChanged()
// 'a' (new) → skip → 'b' (new) → skip → 'a' (new again)
// Result: a, b, a

An important detail

Only neighbors are compared. A value that appeared earlier, then changed and came back, counts as new. Example: a, a, b, a → a, b, a (the third a gets through because the previous output was b).

How values are compared

By default it uses strict equality (===). For objects that means comparison by reference. If you need to compare by a field, there's the form distinctUntilChanged((a, b) => ...).

const { from, distinctUntilChanged } = Rx;

const query$ = from(['ng', 'ng', 'rxjs', 'rxjs', 'ng']).pipe(
  distinctUntilChanged()
);

query$.subscribe(value => {
  console.log('Query: ' + value);
});