JavaScript Array_group Guide
Object.groupBy and Map.groupBy group array elements by a key function, providing SQL-like GROUP BY in JavaScript. The guide walks through Array Grouping. `Object.groupBy(arr, fn)` — group into object. `Map.groupBy(arr, fn)` — group into Map. Example: `Object.groupBy(employees, e => e.department)`. Returns: `{ IT: [emp1, emp2], HR: [emp3] }`. Map.groupBy for non-string keys. Use for: categorizing data, creating lookup tables, dashboard aggregations. 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.