JavaScript Array_reduce Guide
The reduce method iterates over an array, accumulating a single result. It is one of the most powerful array methods. The guide walks through Reduce Basics. `arr.reduce((acc, val, idx, arr) => newAcc, initialValue)`. Sum: `[1,2,3].reduce((a,b) => a+b, 0)`. Flatten: `[[1],[2]].reduce((a,b) => a.concat(b))`. Group: `arr.reduce((g, item) => { (g[item.key]??=[]).push(item); return g }, {})`. Count: `arr.reduce((c, v) => { c[v] = (c[v]||0)+1; return c }, {})`. The guide is organized into 1 sections that build on each other, each pairing a prose explanation with runnable code. Related guides: Arrays, Strings, Numbers, Math, Date. This is one of 145 JavaScript guides on JS Reference — see the full list on the guides page. Practice what you learn in the JavaScript playground, and look up any unfamiliar term in the JavaScript glossary.