/**
 * Insert items into specific index of source.
 *
 * ```ts
 * insertIndex([1, 2, 3], 0, [4, 5]) => [4, 5, 1, 2, 3]
 * insertIndex([1, 2, 3], 1, [4, 5]) => [1, 4, 5, 2, 3]
 * insertIndex([1, 2, 3], 2, [4, 5]) => [1, 2, 4, 5, 3]
 * ```
 */
export declare function insertIndex<T>(source: T[], index: number, items?: T[]): T[];