JavaScript slice: Extract an Array Window Without Editing It

The task

Create the second two-item window from four page records. Then inspect the final item using a negative start and change a field through the extracted window to reveal reference sharing.

Runnable Node 22 example

const assert = require('node:assert/strict');

const pages = [{id: 1}, {id: 2}, {id: 3}, {id: 4}];
const window = pages.slice(2, 4);
assert.deepEqual(window.map(page => page.id), [3, 4]);
assert.equal(pages.length, 4);
assert.notEqual(window, pages);
assert.equal(window[0], pages[2]);
assert.deepEqual(pages.slice(-1).map(page => page.id), [4]);
assert.deepEqual(pages.slice(3, 2), []);
window[0].id = 30;
assert.equal(pages[2].id, 30);
console.log(JSON.stringify(window.map(page => page.id)));

Expected output and explanation

The slice starts at index two and stops before index four, so it initially contains IDs three and four. A reversed range produces an empty array; it does not automatically reverse the data. A start of −1 selects the last element.

Changing window[0].id also changes the third source record because both arrays point to that object. This does not mean slice inserted or removed a source element: the source length remains four. Container independence and nested-record independence are separate properties.

Pitfall to check

Use splice for a deliberate edit and a separate cloning strategy when nested records must be independent. Pagination also needs a defined page-number convention; this example uses zero-based array offsets directly, not a user-facing page-number formula.