TakeWhile & TakeLast in Angular
Introduction
In Angular, TakeWhile and TakeLast are two operators that can be used with RxJS Observables to manipulate the emitted values. These operators take in a predicate function as an argument and return a modified Observable with filtered values.
TakeWhile Operator
The TakeWhile operator emits values from an Observable until the predicate function returns false. It continuously checks every emitted value against the condition specified in the predicate function until it returns false
Example
“`
import { Observable } from ‘rxjs’;
import { takeWhile } from ‘rxjs/operators’;
const numbers = Observable.from([1, 2, 3, 4, 5, 6]);
// TakeWhile Operator with an arrow function as predicate
const stream = numbers.pipe(takeWhile((value) => value < 4));
stream.subscribe((value) => {
console.log(value);
});
// Output:
// 1
// 2
// 3
“`
In the above example, the TakeWhile operator emits values from the Observable until the arrow function `(value) => value < 4` returns false. After 3 is emitted, the arrow function returns false, and the operator stops emitting values.
TakeLast Operator
The TakeLast operator emits the last n values from an Observable stream. The number of values to be emitted is specified as an argument to the operator.
Example
“`
import { Observable } from ‘rxjs’;
import { takeLast } from ‘rxjs/operators’;
const numbers = Observable.from([1, 2, 3, 4, 5, 6]);
// TakeLast Operator with the number of values to be emitted as argument
const stream = numbers.pipe(takeLast(3));
stream.subscribe((value) => {
console.log(value);
});
// Output:
// 4
// 5
// 6
“`
In the above example, the TakeLast operator emits the last three values from the Observable stream.
FAQs
Q. Why use TakeWhile and TakeLast in Angular?
In Angular applications, these operators are helpful when dealing with real-time data streams. They make it easy to filter out irrelevant data and process the necessary data, reducing the processing time and enhancing application performance.
Q. In which cases should I use TakeWhile and TakeLast?
TakeWhile can be used in the case where you want to stop a stream as soon as a certain condition is met. TakeLast is useful when you want to retrieve a specified number of elements from the end of the stream.
Q. Which version of Angular supports TakeWhile and TakeLast operators?
These operators are supported in Angular 6 and above.
Q.What arguments does TakeLast operator take?
TakeLast operator takes in a number (n) as an argument which is the number of elements to be emitted from the end of the stream.
Q.Can I use TakeWhile and TakeLast together?
Yes, you can use TakeWhile and TakeLast together to filter data using both criteria i.e. from the end of the stream and stop when a particular condition is met.