如何在JSON中表达类型？

Question

I want to store tree data using JSON, but I can't figure out how to express types nicely.

Below is how the XML for a hypothetical game involving cats and mice could look like:

<level>
  <entities>
    <cat>
      <direction>right</direction>
      <position x="100" y="100"/>
    </cat>
    <mouse>
      <direction>right</direction>
      <position x="50" y="50"/>
    </mouse>
  </entities>
</level>

The best JSON I can come up with for this is as follows:

{
  "entities": [
    {
      "type": "cat",
      "direction": "right",
      "position": {"x": 100, "y": 100}
    },
    {
      "type": "mouse",
      "direction": "right",
      "position": {"x": 50, "y": 50}
    }
  ]
}

Here I specify the type explicitly. It seems like XML simply is a better fit for this, but unfortunately, I can't really use it, so I'm wondering if there's a nicer way to do it with JSON.

Inspired by Lua's table constructors, I came up with the following, which is clearly not JSON, but JS is actually fine in my case (this data is not intended to be transfered, it's just for a tree of objects):

{
  "entities": [
    cat({
      "direction": "right",
      "position": {"x": 100, "y": 100}
    }),
    mouse({
      "direction": "right",
      "position": {"x": 50, "y": 50}
    })
  ]
};

But, as in Lua, the downside is that I actually have to define all possible entities as functions somewhere:

function cat(options) {
  options.type = "cat";
  return options;
}

function mouse(options) {
  options.type = "mouse";
  return options;
}

What do you think, is there a better way to express types in JSON than to have an attribute for that? Is my hack going too far?

Answer 1

If your types are unique, you could use this

var entities = {
    cat : {
        direction: "right",
        position: {x: 100, y: 100}
    },
    mouse : {
        direction: "right",
        position: {x: 50, y: 50}
    }
};

Then if you want to access different types, use this

Object.keys(entities) // outputs ["cat","mouse"]

EDIT after your comment :

What about this way ?

var entities = {
    cat : [{
        direction: "right",
        position: {x: 100, y: 100}
    },{
        direction: "top",
        position: {x: 150, y: 150}
    }],
    mouse : [{
        direction: "right",
        position: {x: 50, y: 50}
    }]
};

Answer 2

Perhaps you should rethink your XML:

<entities>
  <cat>
   <direction>right</direction>
   <position x="100" y="100"/>
  </cat>
  <mouse>
   <direction>right</direction>
   <position x="50" y="50"/>
  </mouse>
</entities>

Would be better expressed as:

<entities>
  <animal>
   <tpye>cat</type>
   <direction>right</direction>
   <position x="100" y="100"/>
  </animal>
  <animal>
   <tpye>cat</type>
   <direction>right</direction>
   <position x="50" y="50"/>
  </animal>
</entities>

Which could be represented by:

[{"type":"cat","direction":"right", position:{"x":"50","y":50"}},{"type":"mouse","direction":"right", position:{"x":"50","y":50"}}]

Assuming, that you have an "array of animals".