Declarative implementation of Array.prototype.splice

splice.js

/**
 * Simplified `splice` implementation that doesn't handle all the different use cases for `start`.
 * 
 * Deleting items in-place without using `splice` is hard. Here's the plan:
 * 		1. Create an array (`deletedItems`) of items which the user WANTS to remove from the array.
 * 		2. Create an array (`reinsertedItems`) of all items that come AFTER the deleted items.
 * 		3. Truncate the whole array to everything UP TO `start`.
 * 		4. Insert any new items provided by the user.
 * 		5. Reinsert everything in `reinsertedItems`.
 * 		6. Return `deletedItems`.
 * 
 * @param {number} start index to start deleting/inserting at
 * @param {number} [deleteCount] how many items to delete
 * @param {...any} [itemsToInsert] items to insert into the array at `start`
 * @returns {Array} array of deleted items, or an empty array if no items were deleted.
 */
Array.prototype._splice = function(start, deleteCount, ...itemsToInsert) {
	let deletedItems = [];
	let reinsertedItems = [];

	// Parameter setup
	if (!Number.isInteger(deleteCount)) {
		deleteCount = this.length - start;
	}

	let i = start;

	// 1. Create an array (`deletedItems`) of items which the user WANTS to remove from the array.
	for (; i < start + deleteCount; i++) {
		deletedItems.push(this[i]);
	}

	// 2. Create an array (`reinsertedItems`) of all items that come AFTER the deleted items.
	for (; i < this.length; i++) {
		reinsertedItems.push(this[i]);
	}

	// 3. Truncate the whole array to everything UP TO `start`.
	this.length = start;

	// 4. Insert any new items provided by the user
	if (Array.isArray(itemsToInsert)) {
		for (let j = 0; j < itemsToInsert.length; j++) {
			this.push(itemsToInsert[j]);
		}
	}

	// 5. Reinsert everything in `reinsertedItems`.
	for (let k = 0; k < reinsertedItems.length; k++) {
		this.push(reinsertedItems[k]);
	}

	// 6. Return `deletedItems`.
	return deletedItems;
}

const fruits1 = ['Apple', 'Cherry', 'Banana', 'Plum', 'Tangerine', 'Watermelon'];
const removed1 = fruits1._splice(4);
console.log(fruits1, removed1);

const fruits2 = ['Apple', 'Cherry', 'Banana', 'Plum', 'Tangerine', 'Watermelon'];
const removed2 = fruits2._splice(4, 1);
console.log(fruits2, removed2);

const fruits3 = ['Apple', 'Cherry', 'Banana', 'Plum', 'Tangerine', 'Watermelon'];
const removed3 = fruits3._splice(2, 3, 'Peach', 'Nectarine');
console.log(fruits3, removed3);