Extended standard types and Symbol.toStringTag

Might be a question more than a bug, but how would you expect this to react?

const type = require('type-detect');
class ExtendedArray extends Array {
  get [Symbol.toStringTag]() {
    return 'ExtendedArray';
  }
}
console.log(type(new ExtendedArray())); //=> 'array'

My initial expectation was that I would receive back ExtendedArray through the use of Symbol.toStringTag, but the code returns array, from here. Is this intended?

[dvlsg]

Hm. Here’s the official spec for “Array exotic objects”.

I poked around at a couple options, one of which was this, kept me at about 24,000,000 ops/sec (using Node v6.0.0 for this, by the way):

if (isArrayExists && Array.isArray(obj)) {
  if (symbolToStringTagExists === false || typeof obj[Symbol.toStringTag] === 'undefined') {
    return 'array';
  }
  // return the toString.call(obj) + trims/lowercase from here as a shortcut?
}

This works too, but drops down to about 2,800,000 ops/sec for array literals on my machine:

if (isArrayExists && Array.isArray(obj)) {
  var arrayType = Object.prototype.toString.call(obj);
  if (arrayType === '[object Array]')
    return 'array';
}

I also tried dropping the Array.isArray() call entirely, and instead let it flow to the objPrototype section to see if objPrototype === Array.prototype. That got me about 18,000,000 ops/sec.

The first option above did allow me to drop through to the bottom toString.call() and receive back extendedarray. Here’s what I got with a few quick checks:

type(new ExtendedArray()); //=> 'extendedarray'
type([]); //=> 'array'
type(new Array()); //=> 'array'
type(new Uint8Array()); //=> 'uint8array'

Trying to think if that would be sufficient, or if I’m missing a scenario…

Another side note – is it intended that the custom toStringTag returns are lowercase?

[meeber]

imo Symbo.toStringTag should take priority and return “ExtendedArray”, which would make this a bug.

@keithamus @lucasfcosta ?