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.
