In functional programming, being able to reduce is one of the fundamental concepts.

Reduce is used to iterate through the values in an array and condense all of the values into a single value.

Usually the value is a sum of all of the elements, but it could be used to decrement, multiply or even divide.

What is Reduce used for?

Normally, you would want to use a reduce method when you want to sum all of the values in an array and then use the total for some operation. For example, it might make sense to use a reducer when you want to get the total number of items in a shopping cart and determine the tax or determine how many items a customer purchased.

I like using reducers to avoid having to write boiler plate code for loops and being able to do really simple one line operations.

How to Use Reduce

A reduce method takes in a function as a parameter, and that function needs to take in an accumulator and the current element.

So, basically, a really simple example of how to use reduce is this:

var numberArray = [0,1,2,3]

var accumulatedResult = numberArray.reduce(function (total, currentElement) {
	return total + currentElement;
});

If we ran this in the developer console of Chrome, we would see this:

As I mentioned, every element of the array is added together and then the total is available in the accumulatedResult variable. We could write this as a for loop and get exactly the same result if we wanted to but it would be more code:

var numberArray = [0,1,2,3]
var accumulatedResult = 0;

for (var i = 0; i < numberArray.length; i++) {
	accumulatedResult += numberArray[i];
}

And, if we ran this for code for a loop in the developer console, we would see this:

So this means, that a reduce method cycles through the values in an array – in a way similar to a for loop but it’s able to do some mathematical operations like summing of values and store a lot less variables in code and use a lot less boiler plate.

Advantages of JavaScript’s Reduce Method

• the reduce method is that it can handle array elements that don’t have a value. Technically the for loop above would crash if there was a null or undefined value.
• Reduce methods are usually much shorter – and much easier to understand.
• Reduce methods can be chained together – forEach and for loops can’t be easily which means they can’t really be used too well in functional programming.

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Also published on Medium.