What are JavaScript arrays and their methods?

JavaScript arrays are ordered, zero-indexed, dynamic collections. Essential methods: Mutating: push()/pop() (add/remove from end), unshift()/shift() (add/remove from front), splice(start, deleteCount, ...items) (add/remove anywhere), sort(compareFn), reverse(), fill(). Non-mutating (return new array): slice(start, end), concat(), flat(depth), flatMap(), map(), filter(). Search: indexOf(), lastIndexOf(), find() (first match), findIndex(), includes(). Test: some() (any match?), every() (all match?). Iterate: forEach(), entries(), keys(), values(). Reduce: reduce(), reduceRight(). Create: Array.from(), Array.of(), Array(n).fill(). Join to string: join(separator).