MergeArray.ts

import type { UnknownArray } from '../Base/UnknownArray'

export type MergeArrayOptions = {
 mode?: 'replace' | 'spread'
}

export type MergeArray<
 Target extends UnknownArray,
 Source extends UnknownArray,
 Options extends MergeArrayOptions = { mode: 'spread' }
> = (
 [Source | Target] extends UnknownArray
 ? Options['mode'] extends 'replace'
 ? [Target, Source] extends [Readonly<[unknown, ...infer TTail]>, Readonly<[infer SHead, ...infer STail]>]
 ? [SHead, ...MergeArray<TTail, STail, Options>]
 : [...Source, ...Target]
 : [...Target, ...Source]
 : never
)