I need to know if any JSON implementations can handle sparse arrays to my satisfaction. I've seen the question: How to represent a sparse array in JSON? but using an object rather than an array is not an option for me; I need an array.

My minimum requirement would be that the implementation fill in any gaps with "undefined". Otherwise I am writing defensive code that fills in the gaps myself, before JSON encoding.

I need to know if any JSON implementations can handle sparse arrays to my satisfaction. I've seen the question: How to represent a sparse array in JSON? but using an object rather than an array is not an option for me; I need an array.

My minimum requirement would be that the implementation fill in any gaps with "undefined". Otherwise I am writing defensive code that fills in the gaps myself, before JSON encoding.

• javascript
• arrays
• json
• sparse-array

2 Answers 2

Not possible. Forget implementations, it's just plain not allowed in the spec.

http://json/

Arrays are defined by value only. Objects are for when the index/key has meaning.

Could you use an object where the property name was an index and the property value was the value, then run it through an intermediary function to create your sparse array?

function getSparseArray(obj) {
  var ary = [];
  for (prop in obj) {
    var i = parseInt(prop,10);
    if (!isNaN(i)) {
      ary[i] = obj[prop];
    }
  }
  return ary;
}

You would send it something like

{ "5":"Five", "11":"Eleven", "99":"Ninety-Nine"}

and get back an array that was populated with just three values:

ary[5] = "Five"
ary[11] = "Eleven"
ary[99] = "Ninety-Nine"
ary[0] = 'undefined'
ary[98] = 'undefined'
etc.

ary here would have a length of 100, but it would be a "sparse" array in your sense.