Add support for toJSON()

[matthieusieben]

I want encode() to apply the same logic as JSON.stringify(): if the input is an object that has a toJSON function; call it and use the returned value for encoding, as specified in the ecmascript spec.

I tried adding a special logic in typeEncoders.Object:

      if ('toJSON' in obj && typeof (obj as any).toJSON === 'function') {
        const value = (obj as any).toJSON()
        const encoded = encode(value, encodeOptions)
        return [new Token(Type.map, encoded)]
      }

But I have several issues:

1. encode calls defaultWriter.reset(), preventing nested calls to encode()
2. I need to avoid infinite loops, which means that I have to use two different versions of Object custom encoder, so that the toJSON is called only once (if the object returned the first time also has a toJSON method)

How do you suggest I go about doing this?

[rvagg]

I just exposed objectToTokens in my last change, so this should work for you now I think:

import { encode, objectToTokens } from 'cborg'

const bytes = encode(value, {
  typeEncoders: {
    Object: (obj, _typ, options, refStack) => {
      if (typeof obj.toJSON === 'function') {
        return objectToTokens(obj.toJSON(), options, refStack)
      }
      return null // fall through to the default Object encoder
    }
  }
})