Using a JSON.parse reviver to obfuscate fields

Question

I am attempting to abuse a reviver function with JSON.parse.

I basically want to make certain fields "null".

If I do this:

var json_data = JSON.parse(j, function(key, value) {
  if (key == "name") {        
    return value;
  } else {
    return null;    
  }    
});

The entire json_data object ends up null. In fact, no matter what I make the else, that defines the value of the json_object.

Interestingly, this works as expected:

var json_data = JSON.parse(j, function(key, value) {
  if (key == "name") {        
    return "name";
  } else {
    return value;    
  }    
});

The property "name" now has a value of "name".

JSON in question:

var j = '{"uuid":"62cfb2ec-9e43-11e1-abf2-70cd60fffe0e","count":1,"name":"Marvin","date":"2012-05-13T14:06:45+10:00"}';

Update

I just realized that the inverse of what I want to do works as well so I can nullify the name field:

var json_data = JSON.parse(j, function(key, value) {
  if (key == "name") {        
    return null;
  } else {
    return value;    
  }    
});

Answer 1

Through some experimentation, it looks like a final call is made to the function where the key is an empty string and the value is the top-level object:

> JSON.parse('{"hello": "world"}', function(k, v) { console.log(arguments); return v; })
["hello", "world"]
["", Object]

So you could use:

var json_data = JSON.parse(j, function(key, value) {
  if (key == "name" || key === "") {        
    return value;
  } else {
    return null;    
  }    
});

Now, since "" does appear to be a valid JSON key, to be 100% correct it might be better to use something like:

var json_data;
JSON.parse(j, function(key, value) {
  if (key == "name") {        
    return value;
  } else if (key === "") {
    json_data = value;
    return null;
  } else {
    return null;    
  }    
});

But that might be a little bit paranoid ;)

Answer 2

It has a rather interesting behavior that the entire object is included in the objects passed to the reviver.

When the entire object is passed, the key is null.

http://jsfiddle.net/sGYGM/7/

var j = '{"uuid":"62cfb2ec-9e43-11e1-abf2-70cd60fffe0e","count":1,"name":"Marvin","date":"2012-05-13T14:06:45+10:00"}';

var json_data = JSON.parse(j, function(k, v) {
    if (k === "" || k == "name") {
        return v;
    } else {
        return null;
    }
});

console.log(json_data);

As per https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/JSON/parse

The reviver is ultimately called with the empty string and the topmost value to permit transformation of the topmost value. Be certain to handle this case properly, usually by returning the provided value, or JSON.parse will return undefined.