How do I count javascript object's attributes?

Question

Suppose I have the following object in javascript:

var object = {"key1":"value1","key2","value2","key3","value3"};

How do I find out how many tuples I have in the dictionary?

Answer 1

This was answered pretty well in another post: http://stackoverflow.com/questions/126100/how-to-efficiently-count-the-number-of-keys-properties-of-an-object-in-javascript

Answer 2

You can iterate over the object to get the keys or values:

function numKeys(obj)
{
    var count = 0;
    for(var prop in obj)
    {
        count++;
    }
    return count;
}

It looks like a "spelling mistake" but just want to point out that your example is invalid syntax, should be

var object = {"key1":"value1","key2":"value2","key3":"value3"};

Answer 3

There's no easy answer, because Object -- which every object in JS derives from -- includes many attributes automatically, and the exact set of attributes you get depends on the particular interpreter and what code has executed before yours. So, you somehow have to separate the ones you defined from those you got "for free".

Here's one way:

var foo = {"key1": "value1", "key2": "value2", "key3": "value3"};
Object.prototype.foobie = 'bletch'; // add property to foo that won't be counted

var count = 0;
for (var k in foo) {
    if (foo.hasOwnProperty(k)) {
       ++count;
    }
}
alert("Found " + count + " properties specific to foo");

EDIT: The second line shows how other code can add properties to all Object derivatives. If you remove the "hasOwnProperty()" check inside the loop, the property count will go up to at least 4. On a page with other JS besides this code, it could be higher than 4, if that other code also modifies the Object prototype.

Answer 4

This function makes use of Mozilla's __count__ property if it is available as it is faster than iterating over every property.

function countProperties(obj) {
  var count = "__count__",
  hasOwnProp = Object.prototype.hasOwnProperty;

  if (typeof obj[count] === "number" && !hasOwnProp.call(obj, count)) {
    return obj[count];
  }
  count = 0;
  for (var prop in obj) {
    if (hasOwnProp.call(obj, prop)) {
      count++;
    }
  }
  return count;
};

countProperties({
  "1": 2,
  "3": 4,
  "5": 6
}) === 3;