JavaScript map, filter and reduce Explained

In JavaScript, map() transforms every item in an array and returns a new array of the same length, filter() returns a new array with only the items that pass a test, and reduce() combines all the items into a single value, like a total or an object. None of them change the original array. Below I explain each one with practical examples, show how to chain them, and cover the mistakes that cost me the most debugging time. Every example was run with Node.js 20 (and Node 22 for Object.groupBy), and the comments at the end of each block are the real output. The methods work the same way in every m