Infinite recursion when data contains cyclic dependencies

[znerol]

Given the following data-structure containing cyclic dependencies:

var jsonld = require('jsonld');

var a = {
  "@context": {"@vocab": "http://schema.org"},
  "@id": "http://example.com/a",
  hasPart: []
}
var b = {
  "@context": {"@vocab": "http://schema.org"},
  "@id": "http://example.com/b",
  isPartOf: []
}

a.hasPart.push(b);
b.isPartOf.push(a);

jsonld.flatten(a);

I get the following RangeError:

/tmp/node_modules/jsonld/js/jsonld.js:5836
  return (Object.prototype.toString.call(v) === '[object Object]');
                                    ^
RangeError: Maximum call stack size exceeded

Instead I'd expect the following output:

{
  "@context": {
    "@vocab": "http://schema.org/"
  },
  "@graph": [
    {
      "@id": "http://example.com/a",
      "hasPart": {
        "@id": "http://example.com/b"
      }
    },
    {
      "@id": "http://example.com/b",
      "isPartOf": {
        "@id": "http://example.com/a"
      }
    }
  ]
}

I see the same behavior in pyld, so I'm not sure whether this is an implementation problem or an issue with the API spec.

[znerol]

This also happens when attempting to flatten() a data structure which has been link()ed before:

var jsonld = require('jsonld');

var ctx = {"@vocab": "http://schema.org"}
var input = {
  "@context": ctx,
  "@graph": [
    {
      "@id": "http://example.com/a",
      "hasPart": [{"@id": "http://example.com/b"}]
    },
    {
      "@id": "http://example.com/b",
      "isPartOf": [{"@id": "http://example.com/a"}]
    }
  ]
}

jsonld.link(input, ctx, {}, function(err, linked) {
  jsonld.flatten(linked, ctx);
});

[dlongley]

@znerol, there's an expectation (in the spec) that the input to any JSON-LD algorithm is a JSON object, not a JavaScript object. JSON objects cannot have circular references. In short, don't pass anything generated by link back through any JSON-LD algorithm.

[znerol]

Are there any recommendations on how to unlink a linked structure?

[dlongley]

@znerol, unfortunately, we don't have any API calls to do it in jsonld.js ... you can always write a function that will cycle through an object and track when it hits a cyclical reference and instead replace it with a clone of the data.

There's probably code you can adapt to do that at stackoverflow somewhere. For example: this may help.

And perhaps this npm package could help?: circular-json