The beast that is Array.prototype.reduce

Akshendra Pratap Singh on June 23, 2018

reduce() is an absolute beast of a method when it comes to functional style programming in JavaScript. The more you use it, the more you see use ... [Read Full]
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You can use reduce to implement map, filter, etc. That's why some people call reduce as the most important operation of array.

Besides, just to add on, your example of reducing to a boolean can be replaced with Array.prototype.some method.


Yes, reduce is actually an important building block of FP.


Thanks for reminding me about .some(). I always forget that it exists.


Methods like reduce and map always confuse me because the documentation is ultra simple (like summation) while the tech community adores them for being powerful and versatile...

Your post was the first I've seen to approach it with practical examples while still being approachable to someone learning it for the first time. Great post!


I'm a big fan of reduce but there's often a simpler solution, or at least one which makes for more legible code. For example you can flatten an array of arrays using concat and restructuring e.g.

const a = [[1,2,3,4,5],[1,2,3,4,5]];

const flattened = [].concat(...a);
// flattened will be ... [1, 2, 3, 4, 5, 1, 2, 3, 4, 5]

As a general policy I try to name my reducer function arguments to reflect what they're doing in the specific case rather than just giving them general names (as you do in your examples i.e. map vs acc for the 'previousValue' argument).

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