Handle circular references

[eemeli]

Fixes #81

Essentially, this now works:

const a = ['b']
a.push(a)
const str = YAML.stringify(a)
// &a1
// - b
// - *a1
YAML.parse(str)
// [ 'b', [Circular] ]

For stringifying, this means that input that would earlier throw now doesn't, producing valid YAML instead. When parsing, alias nodes referring to anchors now resolve the original objects, rather than separate instances. I'm not sure yet whether this should mean a major version bump or not.

The public YAML.createNode() and the doc.schema.createNode() APIs have not changed, and do not support the autogeneration of alias nodes.

[isaacs]

Fantastic! I'll play with this tonight and provide some feedback. Thanks!

[isaacs]

As far as I can tell, this works perfectly.

My only complaint is that there doesn't appear to be a way to disable the warning from being printed to stderr.

// dist/Document.js:455
if (!src.resolved) {
  const msg = 'Alias node contains a circular reference, which cannot be resolved as JSON';
  this.warnings.push(new _errors.YAMLWarning(node, msg));
}

// dist/index.js:66
doc.warnings.forEach(warning => console.warn(warning));

If it handles circular references fine, then so what? If I'm using YAML to stringify and parse, who cares that it doesn't make for valid JSON?

[eemeli]

You're right, that warning doesn't make sense any more, so I removed it.