toJSON.js

import mapObject from './mapObject'

const toInfo = proto => name => ({
  name,
  desc: Object.getOwnPropertyDescriptor(proto, name)
})

const hasGetter = ({ desc }) => desc.get != null

const getName = ({ name }) => name

const listGetters = proto => {
  const lookup = Object.create(null)
  while (proto != null && proto !== Object.prototype) {
    const getters = Object.getOwnPropertyNames(proto)
      .filter(elem => elem !== 'constructor')
      .map(toInfo(proto))
      .filter(hasGetter)
      .map(getName)
    for (const name of getters) {
      lookup[name] = true
    }
    proto = Object.getPrototypeOf(proto)
  }
  return Object.keys(lookup)
}

const convertElement = el => '' + el

const convertTimeRanges = ranges => ({
  $type: 'TimeRanges',
  data: [...Array(ranges.length).keys()].map(i => ({
    start: ranges.start(i),
    end: ranges.end(i)
  }))
})

const convertArrayLike = type => (obj, seen) => ({
  $type: type,
  data: Array.prototype.map.call(obj, e => circularToJSON(e, seen))
})

const propertiesToJSON = (o, names, seen) =>
  mapObject(names, name => {
    let value, ok
    try {
      value = o[name]
      ok = true
    } catch (err) {
      value = err
      ok = false
    }
    return ok ? circularToJSON(value, seen) : `[Error: ${value}]`
  })

const converters = [
  { test: o => o == null, convert: () => null },
  { test: o => typeof o !== 'object', convert: o => o },
  {
    test: Array.isArray,
    convert: (a, seen) => a.map(e => circularToJSON(e, seen))
  },
  { test: o => typeof o.nodeName === 'string', convert: convertElement },
  { test: o => o instanceof window.TimeRanges, convert: convertTimeRanges },
  {
    test: o => o instanceof window.TextTrackList,
    convert: convertArrayLike('TextTrackList')
  },
  {
    test: o => o.$type === 'SortedList',
    convert: (o, seen) => circularToJSON(o.toArray(), seen)
  },
  {
    test: o =>
      o.constructor != null && o.constructor.prototype !== Object.prototype,
    convert: (o, seen) =>
      propertiesToJSON(o, listGetters(o.constructor.prototype), seen)
  },
  {
    test: () => true,
    convert: (o, seen) => propertiesToJSON(o, Object.keys(o), seen)
  }
]

const circularToJSON = (value, seen) => {
  // TODO Identify circular refs
  if (seen.includes(value)) {
    return '[Circular]'
  }
  seen.push(value)
  const idx = seen.length - 1
  const json = converters.reduce(
    (res, { test, convert }) =>
      !res.ok && test(res.value)
        ? { ok: true, value: convert(res.value, seen) }
        : res,
    {
      ok: false,
      value
    }
  ).value
  seen.splice(idx, 1)
  return json
}

const toJSON = value => circularToJSON(value, [])

export default toJSON