Reduce is an extremely useful tool for extracting data out of collections of data.

Learn how to use the reduce function on javascript arrays to transform a list of values into something else. In this introduction we'll be taking a list of numbers and reducing them into a sum.

javascript

Transform an Array into a Single Value using Reduce

We'll look at using array.reduce to transform an array of strings into an object that counts the occurrence of each string in the array.

Reduce an Array into a Single Object

Learn how two common array functions - map() and filter() - are syntactic sugar for reduce operations. Learn how to use them, how to compose them, and how using reduce can give you a big performance boost over composing filters and maps over a large data set.

Use Reduce to Filter and Map over Large Datasets

Sometimes we need to turn arrays into new values in ways that can't be done purely by passing an accumulator along with no knowledge about its context. Learn how to reduce an array of numbers into its mathematical mean in a single reduce step by using the optional `index` and `array` reducer arguments.

Use the Optional Reduce Arguments

In this lesson, we will be looking at two common mistakes programmers make when working with `reduce.` We will see why we always need to pass in an initial value for our accumulator as well as return the accumulator.

Avoid Common Mistakes when Working with Reduce

Learn a few advanced reduction patterns: flatten allows you to merge a set of arrays into a single array, the dreaded flatmap allows you to convert an array of objects into an array of arrays which then get flattened, and reduceRight allows you to invert the order in which your reducer is applied to your input values.

Learn to Flatten and Flatmap Arrays with Reduce

Let's take a closer look at using Javascript's built in Array reduce function. Reduce is deceptively simple and when harnessed correctly can achieve very powerful results. By leveraging reduce, we can answer a variety of questions on a single, simple data set. In this lesson, we'll look at how you might use [Array.prototype.reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce) to:

- Sum an array of numbers
- Reduce an array of objects to a sum of a given property
- Group an array of objects by key or a set of given criteria
- Count the number of objects in an array by key or a given set of criteria

Array.prototype.reduce in JavaScript by example

Javascript has first-class functions which means that any function can stored, passed around, and returned just like an Object can. This allows for a lot of flexibility with how you call functions. This allows us to use array reduction to create functional pipelines by composing arrays of functions.

Compose Functions with Reduce

A common problem when dealing with some kinds of data is that not every object has the same nested structure. luke_skywalker.parents.father.is_jedi works, but anakin_skywalker.parents.father.is_jedi throws an exception, because anakin_skywalker.parents.father is undefined. But we can reduce a path to provide safe default values and avoid exceptions when walking the same path on non-homogenous objects - watch to learn how! :)

...understand how to use reduce

Joel Hooks

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