arccore.util.deepCopy is not general-purpose

[ChrisRus]

arccore.util.deepCopy (aka arccore.util.clone) derives from an implementation taken from the CoffeeScript Cookbook. This function has generally performed well. But, it's not well tested. And, it's not adequately documented...
... Specifically, it does not appear that this function handles arrays correctly. And, all manner of hell can break loose if the input reference is to something with internal cycles.
Take another look at this:

Should work for arrays (add tests)
Should not allow stack overflow due to unbounded recursion

Here's a little code fragment I authored that's related to arccore.util.deepCopy (used in the context of trying to clone DirectedGraph class instances).

const arccore = require("@encapsule/arccore");

var response = arccore.graph.directed.create({
    name: "Source Digraph",
    description: "This is a graph that we will clone.",
    vlist: [
        { u: "apple", p: "Edible skin" },
        { u: "orange", p: "Inedible skin" }
    ],
    elist: [
        { e: { u: "apple", v: "orange" }, p: "Not the same" }
    ]
});
if (response.error) {
    throw new Error(response.error);
}
var sourceDigraph = response.result;

// Clone via serialize/deserialize
response = arccore.graph.directed.create(sourceDigraph.toJSON());
if (response.error) {
    throw new Error(response.error);
}
var cloneDigraph1 = response.result;

// Clone via arccore.util.clone
var cloneDigraph2 = arccore.util.clone(sourceDigraph);

// Compare contents of the source and clone 1 digraphs. We expect them to be the same.
if (sourceDigraph.stringify() !== cloneDigraph1.stringify()) {
    console.log("! Source digraph JSON is not equal to clone 1 digraph JSON.");
} else {
    console.log("> Clone 1 digraph JSON reads as expected.");
}

// Compare contents of the source and clone 2 digraphs. We expect them to be the same.
if (sourceDigraph.stringify() !== cloneDigraph2.stringify()) {
    console.log("! Source digraph JSON is not equal to clone 2 digraph JSON.");
} else {
    console.log("> Clone 2 digraph JSON reads as expected.");
}

// What's not 100% clear is what happens when we mutate the containers. Specifically,
// client code could subsequently mutate the source, or derived clone in a number of
// different ways: CRUD operation on vertex/edge properties, topological updates
// (vertex & edge add/remove operations).
//
// So the questions I've come up with so far are:
// - How best to affect a deep copy of a DirectedGraph instance.
// - Is it useful to have deep copy for topology and shallow (i.e. shared) copy for vertex/edge properties?
// - What exactly does arccore.util.clone and arccore.util.deepcopy do? Do they act as we expect? How about for DirectedGraph?

// Let's assume that clone 1 is a true deep copy meaning that we can change both its properties and topology without any impact on the source or other test clone.

// Update a vertex property on the clone 1
cloneDigraph1.setVertexProperty({ u: "apple", p: "apple property updated specifically on clone 1" });

if (sourceDigraph.getVertexProperty("apple") === cloneDigraph1.getVertexProperty("apple")) {
    console.log("! Update to clone 1 vertex property reflected back to source digraph.");
} else {
    console.log("> Update of clone 1 vertex property did not update source digraph.");
}

cloneDigraph1.addEdge({ e: { u: "apple", v: "mango" }, p: "Better together..." });

if (sourceDigraph.isEdge({ u: "apple", v: "mango"})) {
    console.log("! Edge added to clone 1 reflected back to source digraph.");
} else {
    console.log("> Edge added to clone 1 did not update source digraph.");
}

// clone 2
cloneDigraph2.setVertexProperty({ u: "apple", p: "apple property updated specifically on clone 1" });

if (sourceDigraph.getVertexProperty("apple") === cloneDigraph2.getVertexProperty("apple")) {
    console.log("! Update to clone 2 vertex property reflected back to source digraph.");
} else {
    console.log("> Update of clone 2 vertex property did not update source digraph.");
}

cloneDigraph2.addEdge({ e: { u: "apple", v: "mango" }, p: "Better together..." });

if (sourceDigraph.isEdge({ u: "apple", v: "mango"})) {
    console.log("! Edge added to clone 2 reflected back to source digraph.");
} else {
    console.log("> Edge added to clone 2 did not update source digraph.");
}

console.log("----------------------------------------------------------------");
console.log("Conclusions:");
console.log("- Deep copy by calling toJSON on source digraph passing the resultant object to arccore.graph.directed.create.");
console.log("- DO NOT use arccore.util.clone until this funciton is tested and documented. And, do not use its alias arccore.util.deepCopy because it's a lie in v0.1.3 apparently.");