JavaScript Set: Track Membership and Inspect Object Identity

The task

Track active badge labels, repeat one addition, remove a member and compare two separate record objects with the same id.

Runnable Node 22 example

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

const active = new Set();
active.add('badge-A').add('badge-B').add('badge-A');
assert.equal(active.size, 2);
assert.equal(active.has('badge-A'), true);
assert.equal(active.delete('badge-A'), true);
assert.equal(active.delete('badge-A'), false);
assert.equal(active.has('badge-A'), false);
const first = {id: 1};
const objects = new Set([first, {id: 1}]);
assert.equal(objects.size, 2);
assert.equal(objects.has(first), true);
assert.equal(objects.has({id: 1}), false);
console.log(`active=${active.size}; objects=${objects.size}`);

Expected output and explanation

Adding badge-A twice does not increase the primitive membership count. The first deletion reports success and the second reports that there was no remaining matching value to remove. The final active size is one.

The object set retains two references. Reusing first finds a member, but constructing another object does not find one merely because id is equal. If membership is defined by an id field, store or index that key deliberately. The separate duplicate-removal exercise defines a first-record-per-id policy.

Pitfall to check

A Set membership check is not a permission system by itself. The application must decide who can alter the tracker and what its values mean. This example is an in-memory data-structure experiment, not an authorization design.