`Rx.Observable.prototype.count([predicate], [thisArg])`

Ⓢ

Returns an observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.

Arguments

[predicate] (Function): A function to test each element for a condition. The callback is called with the following information:
1. the value of the element
2. the index of the element
3. the Observable object being subscribed
[thisArg] (Any): Object to use as this when executing predicate.

Returns

(Observable): An observable sequence containing a single element with a number that represents how many elements in the input sequence satisfy the condition in the predicate function if provided, else the count of items in the sequence.

Example

/* Without a predicate */
var source = Rx.Observable.range(0, 10).count();

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + x.toString());
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    });

// => Next: 10
// => Completed

/* With a predicate */
var source = Rx.Observable.range(0, 10)
    .count(function (x) { return x % 2 === 0; });

var subscription = source.subscribe(
    function (x) {
        console.log('Next: ' + x.toString());
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    });

// => Next: 5
// => Completed