Copy an Array in JavaScript: Test Shallow and Deeper Independence

The task

Create a shallow task-array copy and a structuredClone of the same original, then update their nested completion flags separately. The fixture contains only ordinary arrays, objects, strings and Booleans.

Runnable Node 22 example

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

const tasks = [{name: 'audit', status: {done: false}}];
const shallow = tasks.slice();
const independent = structuredClone(tasks);
assert.notEqual(shallow, tasks);
assert.equal(shallow[0], tasks[0]);
assert.notEqual(independent[0].status, tasks[0].status);
shallow[0].status.done = true;
assert.equal(tasks[0].status.done, true);
assert.equal(independent[0].status.done, false);
independent.push({name: 'file', status: {done: false}});
assert.equal(tasks.length, 1);
console.log(`originalDone=${tasks[0].status.done}; clonedDone=${independent[0].status.done}; clonedSize=${independent.length}`);

Expected output and explanation

The shallow copy passes the container-identity check but shares the task record, so its nested edit reaches the original. The separately cloned status remains false. Adding a task to the independent array also leaves the original membership unchanged.

The assertions address two different changes: editing a shared child and adding an outer element. Testing only array length would miss the nested-sharing problem. structuredClone is available in this Node22 example, but its supported-value contract is not “every possible JavaScript object.”

Pitfall to check

Functions and some other values cannot be cloned through structuredClone. Do not generalize the fixture to objects with methods, special identity requirements or external resources without checking their semantics. JSON serialization is not an equivalent universal substitute.