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In this lecture, we were going to learn how to accumulate values with the Reduce operator, the Reduce

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operator is the counterpart to the array reduce function.

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If you're familiar with the reduced function on arrays, understanding the Reduce operator will be a

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breeze.

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In some cases, we may want to accumulate the values emitted from an observable.

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The Reduce operator accomplishes this action.

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Let's say our observable emitted the values one through five.

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The observer may want the sum of these values.

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Instead of letting the observer handle this computation, we can add the reduced function to our pipeline

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every time a value is emitted.

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The Reduce operator will take in the value until the next value is emitted.

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Once the observable completes the reduce operator, it will push a value onto the observer throughout

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this entire process.

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The observer will receive a single value.

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We can modify how values are accumulated by passing in a function.

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In this example, we are adding the values together.

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Let's dive into the arguments through our code.

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For this example, let's create a stream of numbers.

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This stream will be reduced to a single value with the Reduce operator.

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First, we will update the import statement for the creation operators.

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We will import the of operator.

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As we learned, the Earth operator will allow us to create a synchronous stream of data next completely

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replace the from event operator with the operator.

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The other operator will limit numbers one through five.

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As it stands, the numbers will be emitted one by one, we can intercept each value with the reduce

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operator to accumulate them.

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Let's give it a try by importing the Reduce operator from the Oryx SJS slash operators package.

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In the pipeline, replace the current operators with the Reduce operator.

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The Reduce operator has two arguments, which are a function and seed value.

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Let's pass in an empty function and a seed value of zero.

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The seed value is optional, but typically you will want to add it.

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This value refers to the starting value.

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The goal is to get the sum of the values emitted from the observable.

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Our starting value should start at zero.

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New numbers are added to the seed value.

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Next, we can start accumulating the values through the arrow function.

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This function will be supplied with two arguments called accumulator and value.

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The accumulator argument stores the value previously returned by this function, if this function is

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being called for the first time, the accumulator argument will store the seed value.

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As for the value argument, it references the value emitted by the observable.

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We must return a value since we're trying to get the sum.

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We all have these two arguments together.

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In the end, we will receive the sum in the console, the behavior of the reduced operator is entirely

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different from other operators.

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Typically, each time a value is pushed, the value would get lost in the console.

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However, in this scenario, the reduced operator does not push each value emitted from the observable.

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It's waiting until the observable has been completed.

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After the observable completes, the reduce operator will push the value it has accumulated over time.

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This operator does introduce some problems.

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What if the observable it's reading from doesn't complete?

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Let's look at how we can tackle this scenario in the following lecture.