JavaScript Array filter: Select Records Without Cloning Them

The task

Select available workshop supplies and verify what happens when a field on a selected record changes. Keep the source membership and object-content questions separate.

Runnable Node 22 example

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

const stock = [{id: 'clip', quantity: 4}, {id: 'pad', quantity: 0}, {id: 'pen', quantity: 2}];
const available = stock.filter(item => item.quantity > 0);
assert.deepEqual(available.map(item => item.id), ['clip', 'pen']);
assert.equal(stock.length, 3);
assert.equal(available[0], stock[0]);
available[0].quantity = 3;
assert.equal(stock[0].quantity, 3);
assert.deepEqual(stock.filter(item => item.quantity > 100), []);
console.log(available.map(item => item.id).join(','));

Expected output and explanation

The zero-quantity pad is not selected, so the result has two members while stock still has three. Updating the retained clip record changes the same object visible through stock; filter did not create an independent record copy.

The final check returns an empty array because no quantity exceeds one hundred. That result is a valid selection outcome, not a missing return or a thrown error. A caller can inspect length to decide how to present an empty result.

Pitfall to check

If a callback uses braces, explicitly return the condition. A missing return yields undefined and can filter out every element. Validate quantity types separately if external records may contain text or missing fields.