Create Map from Object which has length Property

[jaganathanb]

There is a question in SO which talks about custom fromJS

fromJS is taken from the Immutable docs page

function fromJS(js) {
    return typeof js !== 'object' || js === null ? js :
        Array.isArray(js) ? 
            Immutable.Seq(js).map(fromJS).toOrderedSet() :
            Immutable.Seq(js).map(fromJS).toMap();
}

var output = fromJS({
    measurements: {
        length: 5,
        weight: 30
    }
}).toJS();

The above code outputs,

//output
[object Object] {
  measurements: [object Object] {
    0: undefined,
    1: undefined,
    2: undefined,
    3: undefined,
    4: undefined
  }
}

But it has to be printed as below.

[object Object] {
  measurements: [object Object] {
    length: 5,
    weight: 30
  }
}

What is wrong with the above custom fromJS function.

JSBin is here

Note: If we change the property length to mLength, it works as expected.

[kitten]

@jaganathanb The problem seems to be here: https://github.com/facebook/immutable-js/blob/master/src/Seq.js#L359

Seq is creating an ArraySeq if the object passed has a property length that is a number, which seems to be bollocks. I'm not sure why it isn't using Array.isArray.

You could just write a version that doesn't use Seq for plain objects:

function fromJS(x) {
  if (Array.isArray(x)) {
    return new List(x).map(fromJS)
  } else if (typeof x === 'object') {
    return Object.keys(x).reduce((acc, key) => (
      acc.set(key, fromJS(x[key]))
    ), new Map().asMutable())
  }

  return x
}

This is of course not optimal, but it should work.

[ShirtlessKirk]

@philpl "I'm not sure why it isn't using Array.isArray"

Because things like NodeLists and arguments aren't arrays, but are array-like. The issue here is that if there is a length property, to truly check if it is array-like the key of length - 1 should be defined on the object:

var o = {
  '4': undefined, // array 'index' property, set to undefined 
  // to muck with simple checks like a[a.length - 1] != undefined. Muhaha.
  length: 5,
  width: 30
}; // o.hasOwnProperty(o.length - 1) === true, so object is truly array-like

[kitten]

@ShirtlessKirk True! I didn't think of those. In that case, I'll open a PR for the extra check.

[leebyron]

In the meantime, Seq.Keyed() will not expect array-likes.