side-channel 
Store information about any JS value in a side channel. Uses WeakMap if available.
Warning: in an environment that lacks WeakMap, this implementation will leak memory until you delete the key.
Getting started
npm install --save side-channel
Usage/Examples
const assert = require('assert');
const getSideChannel = require('side-channel');
const channel = getSideChannel();
const key = {};
assert.equal(channel.has(key), false);
assert.throws(() => channel.assert(key), TypeError);
channel.set(key, 42);
channel.assert(key); // does not throw
assert.equal(channel.has(key), true);
assert.equal(channel.get(key), 42);
channel.delete(key);
assert.equal(channel.has(key), false);
assert.throws(() => channel.assert(key), TypeError);
Tests
Clone the repo, npm install, and run npm test