ljharb
internal-slot 
Truly private storage, akin to the JS spec’s concept of internal slots.
Uses a WeakMap when available; a Map when not; and a regular object in even older engines. Performance and garbage collection behavior will reflect the environment’s capabilities accordingly.
Example
var SLOT = require('internal-slot');
var assert = require('assert');
var o = {};
assert.throws(function () { SLOT.assert(o, 'foo'); });
assert.equal(SLOT.has(o, 'foo'), false);
assert.equal(SLOT.get(o, 'foo'), undefined);
SLOT.set(o, 'foo', 42);
assert.equal(SLOT.has(o, 'foo'), true);
assert.equal(SLOT.get(o, 'foo'), 42);
assert.doesNotThrow(function () { SLOT.assert(o, 'foo'); });Tests
Simply clone the repo, npm install, and run npm test
Security
Please email @ljharb or see https://tidelift.com/security if you have a potential security vulnerability to report.