unbox-primitive 
Unbox a boxed JS primitive value. This module works cross-realm/iframe, does not depend on instanceof or mutable properties, and works despite ES6 Symbol.toStringTag.
Example
var unboxPrimitive = require('unbox-primitive');
var assert = require('assert');
assert.equal(unboxPrimitive(new Boolean(false)), false);
assert.equal(unboxPrimitive(new String('f')), 'f');
assert.equal(unboxPrimitive(new Number(42)), 42);
const s = Symbol();
assert.equal(unboxPrimitive(Object(s)), s);
assert.equal(unboxPrimitive(new BigInt(42)), 42n);
// any primitive, or non-boxed-primitive object, will throw
Tests
Simply clone the repo, npm install, and run npm test