RxJS last() Filtering Operator

by Online Tutorials Library July 22, 2022

RxJS last() Filtering Operator

RxJS last() operator is a filtering operator that emits only the last value or the last value that meets some specified condition emitted by the source observable.

Syntax:

Following is the syntax of the RxJS last() operator:

  last<T, D>(predicate?: (value: T, index: number, source: Observable<T>) => boolean, defaultValue?: D): OperatorFunction<T, T | D>  

Parameter Explanation

predicate: It is used to specify the condition that any source emitted item has to satisfy. It is an optional argument. Its default value is undefined.
defaultValue: It specifies if the last predicate isn’t met or no values were emitted. It is also an optional argument. Its default value is undefined.

Return value

The RxJS last() operator’s return value is observable that emits the last value.

Let us see some examples of the RxJS last() operator to understand it clearly.

Example 1 (Emit last value in a given sequence)

  import { from } from ‘rxjs’;  import { last } from ‘rxjs/operators’;  const source = from([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);  //no arguments, emit last value  const example = source.pipe(last());  const subscribe = example.subscribe(val => console.log(`The last value is: ${val}`));  

Output:

After executing the above example, you will see the following result:

Example 2 (Emit last value to meet a specific condition)

  import { from } from ‘rxjs’;  import { last } from ‘rxjs/operators’;  const source = from([1, 2, 3, 4, 5, 6, 7, 8, 9]);  //emit last even number  const exampleTwo = source.pipe(last(num => num % 2 === 0));  const subscribeTwo = exampleTwo.subscribe(val =>    console.log(`The last value to meet condition is: ${val}`)  );  

Output:

After executing the above example, you will see the following result:

In the above example, you can see that the last given value is 9, but the condition is to emit the last even number; that’s why it has emitted 8.

Example 3 (Emit Last with default value)

  import { from } from ‘rxjs’;  import { last } from ‘rxjs/operators’;  const source = from([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);  //no values will pass given predicate, emit default  const exampleTwo = source.pipe(last(v => v > 10, ‘Nothing to emit’));  const subscribeTwo = exampleTwo.subscribe(val => console.log(val));  

Output:

After executing the above example, you will see the following result:

Next TopicRxJS Operators

RxJS last() Filtering Operator

RxJS last() Filtering Operator

Syntax:

Parameter Explanation

Return value

Example 1 (Emit last value in a given sequence)

Example 2 (Emit last value to meet a specific condition)

Example 3 (Emit Last with default value)

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