JavaScript Reduce — A Complete Guide to the Only JS Array Function You Really Need

While the JavaScript map, filter, and find functions will help you do a lot with JavaScript arrays, every now and then, you have to do more than what these methods can handle. JavaScript reduce is the Swiss Army knife of array manipulation in JavaScript that can not only do what these other functions can't but also everything they can.

Disclaimer: just because you can do all these things with reduce doesn't mean that there are more performant ways of doing the same thing. I have just run into developers who never use reduce because they aren't really sure what it does or think that the syntax is confusing. I was once in the same boat, but once I saw what it could do and used it a few times, it made more sense.

What is the JavaScript reduce Function?

The textbook definition of JavaScript reduce is:

JavaScript reduce is a higher order JavaScript function used for manipulating data that reduces an array to a single value.

But what this definition doesn’t tell you is that this “single value” can be anything: number, string, array, or object. And that definition is what kept me from using it for a long time unless I needed to do something like get the sum of an array of numbers.