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Reactive Programming with RxJS

Reduce, Aggregate and Flatmap Operators of Sequencing

Explore how to apply reduce and aggregate operators to process sequences and compute single values, understand scan for real-time aggregation of infinite streams, and use flatMap to flatten nested Observables into a single output stream.

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Reduce

The reduce operator, also known as fold, takes an Observable and returns a new one that always contains a single item, which is the result of applying a function over each element. This function receives the current element and the result of the function’s previous invocation.

Note: The reduce function has another parameter that is optional and can be used to pass a starting seed value.

var Rx = require('rx');
var src = [1,2,3,4,5];
var sum = src.reduce(function(a,b)
{
  return a+b;
});
console.log(sum);
Using the reduce operator

Now let’s run the code while subscribing to the value:

var Rx = require('rx');
var logValue = function(val) { console.log(val) };
var src = Rx.Observable.range(1, 5);
var sum = src.reduce(function(acc, x) 
{
  return acc + x;
});
sum.subscribe(logValue);
Using the reduce operator and subscribing to it

The reduce operator is a powerful operator that is used to manipulate a sequence. It is, in fact, the base implementation for a whole subset of methods called aggregate operators.

Aggregate operators

Aggregate operators process a sequence and return a single value. For example, Rx.Observable.first takes an Observable and an optional predicate function and returns the first element that satisfies the condition in the ...