Set shallow clone

set-clone-broken.js

const assert = require('assert');
const fs = require('fs');
const vm = require('vm');
const iterateSetElems = set => Set.prototype.keys.call(set);

const Builtin = require('proposal-istypes/polyfill'); // Git dependency
const SetPolyfillSource = fs.readFileSync(require.resolve('es6-shim'), 'utf8');

// Get a good polyfill for Set.
// We achieve this by loading the Set implementation from es6-shim.
// es6-shim doesn't support @@builtin, so we add it on top.
const vmCtx = (() => {
	const sandboxGlobal = {
		require: require, SYMBOL_BUILTIN: Symbol.builtin, module: {exports: {}},
		Set: class BrokenSet {add() {} has() {} delete() {}}, // Required to convince es6-shim to implement its Set polyfill.
	};
	sandboxGlobal.global = sandboxGlobal;
	sandboxGlobal.exports = sandboxGlobal.module.exports;

	const ctx = vm.createContext(sandboxGlobal);
	vm.runInContext(SetPolyfillSource, ctx);
	vm.runInContext(`this.Set[SYMBOL_BUILTIN] = () => 'Set';`, ctx); // Implement Set@@builtin
	return ctx;
})();

const cloneMethods = {
	'Set': function cloneSet(source) {
		return new Set(iterateSetElems(source));
	},
};

function shallowClone(base) {
	if (base === null || typeof base !== 'object') return base;
	const type = Builtin.typeOf(base);
	const clone = type in cloneMethods ? cloneMethods[type](base) : Object.create(Object.getPrototypeOf(base));
	Object.defineProperties(clone, Object.getOwnPropertyDescriptors(base));
	return clone;
}

const realSet = new Set([1, 2]);

vm.runInContext(`
	this.result = new Set([1, 2]);
`, vmCtx);

const polySet = vmCtx.result;

assert(realSet.has(1));
assert(shallowClone(realSet).has(1));

assert(polySet.has(1));
assert(shallowClone(polySet).has(1)); // TypeError exception @shallowClone