RxJS - Creation Operator count

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count() takes in an Observable with values and converts it into an Observable that will give a single value. The count function takes in predicate function as an optional argument. The function is of type boolean and will add value to the output only if the value is truthy.

Syntax

Here is the syntax for Count −

count(predicate_func? : boolean): Observable

Parameters

predicate_func − (optional) Function that will filter the values to be counted from the source observable and return a boolean value.

Return value

The return value is an observable that has the count of the given numbers.

Let us see some examples of count without predicate and with function.

Example 1

The following example is without predicate function −

import { of } from 'rxjs';
import { count } from 'rxjs/operators';

let all_nums = of(1, 7, 5, 10, 10, 20);
let final_val = all_nums.pipe(count());
final_val.subscribe(x => console.log("The count is "+x));

Output

The count is 6

Example 2

The following example is with predicate function −

import { of } from 'rxjs';
import { count } from 'rxjs/operators';
let all_nums = of(1, 6, 5, 10, 9, 20, 40);
let final_val = all_nums.pipe(count(a => a % 2 === 0));
final_val.subscribe(x => console.log("The count is "+x));

The function we have used in the count is to give the count only of even numbers.

Output

The count is 4