Question

Possible Duplicate:
Length of Javascript Associative Array

I want to check the length of a multidimensional array but I get “undefined” as the return. I’m assuming that I am doing something wrong with my code but I can’t see anything odd about it.

alert(patientsData.length); //undefined
alert(patientsData["XXXXX"].length); //undefined
alert(patientsData["XXXXX"]['firstName']); //a name

fruits = ["Banana", "Orange", "Apple", "Mango"];
alert(fruits.length); //4

Thoughts? Could this have something to do with scope? The array is declared and set outside of the function. Could this have something to do with JSON? I created the array from an eval() statement. Why does the dummy array work just fine?

Answer 1

Those are not arrays. They’re objects, or at least they’re being treated like objects. Even if they are Array instances, in other words, the “length” only tracks the largest numeric-indexed property.

JavaScript doesn’t really have an “associative array” type.

You can count the number of properties in an object instance with something like this:

function numProps(obj) {
  var c = 0;
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) ++c;
  }
  return c;
}

Things get somewhat messy when you’ve got inheritance chains etc, and you have to work out what you want the semantics of that to be based on your own architecture.