deep clone

deepClone.js

function deepClone(obj, hash = new Map()) {
    if (Object(obj) !== obj) return obj; // primitives
    if (hash.has(obj)) return hash.get(obj); // cyclic reference
    var result = Array.isArray(obj) ? []
               : obj.constructor ? new obj.constructor() : {};
    hash.set(obj, result);
    if (obj instanceof Map)
        Array.from(obj, ([key, val]) => result.set(key, deepClone(val, hash)) );
    return Object.assign(result, ...Object.keys(obj).map (
        key => ({ [key]: deepClone(obj[key], hash) }) ));
}

// sample data
function A() {}
function B() {}

var a = new A();
var b = new B();

a.b = b;
b.a = a;

// test it
var c = deepClone(a);

console.log('a' in c.b.a.b); // true
Source