Demo: last
The last is a filter operator that emits only the last value or the last value matching the optional condition, when specified.
last(); //Witout any condition
last(v => optionalCondition)
The last operator emits only after the source completes and throws an EmptyError if no match is found.
//Emits every .5 sec starting at .5 sec
const source = timer(500, 500).pipe(take(9));
//emits the last matched item after the source completes
const filterItems = last((v) => v%3==0 );
const output = source.pipe(filterItems).subscribe(v=>console.log(v))What is happening ?
The operators first and last can filter both by position as well as by condition.
The last operator without a condition parameter is like takeLast(1):
- It will emit only the last item from the source when the source completes.
Secondly, the last operator can also take a boolean condition.
- Where it will keep updating the last matching source item in it's buffer.
- And, emit the same as the source completes.
Importantly, if the operator could not find a match to emit:
- It will throw an "EmptyError" when the source completes.
Coming to our demo, the source emits 9 numbers:
- Starting with 0, 1, 2 and continuing upto 8.
- But, as we are interested only on the last number divisible by 3.
- We have piped in our source into the last operator with the desired condition as shown.
- For this, we will only get the last matching value 6 in our output, as the source completes.