var array = [[1, 2, 3],[4, 5, 6]];
alert(array[1][1]);
// Output: 5

Here is the same sample but with another sytnax:

var array = new Array(2);
array[0] = new Array(3);
array[0][0] = 1;
array[0][1] = 2;
array[0][2] = 3;
array[1] = new Array(3);
array[1][0] = 4;
array[1][1] = 5;
array[1][2] = 6;
alert(array[1][1]);
// Output: 5

Again, another syntax of this example to access a javascript array:

var array = new Array(new Array(1, 2, 3), new Array(4, 5, 6));
alert(array[1][1]);
// Output: 5

Some developers make the following error when accessing a JavaScript array and wait that the 5 will be returned:

var array = [[1, 2, 3],[4, 5, 6]];
alert(array[1,1]);
// Output: [4, 5, 6]

JavaScript interpretes 1,1 as a comma separated expression and this expression returnes 1. Each part of the comma separated expression is executed left to right. The final value of the comma separated expression is the value of the last comma part. So, when you try to evaluate the following script in the sample below you will get the last array item because the value of the comma separated expression 0,1,2 will be 2:

var array = [1, 2, 3];
alert(array[0,1,2]);
// Output: 3

Problem

alert({}==null); // false
alert({}=={}); // false
alert({}=={}); // false
alert({}==0); // false
alert({}==""); // false
alert({}==[]); // false
// etc :)

So, none of the expressions above can check for an empty javascript object.

Pure javascript solution

function empty(o) {
    for(var i in o) 
        if(o.hasOwnProperty(i))
            return false;
 
    return true;
}

JQuery + JQuery.JSON solution

var o = {};
alert($.toJSON(o)=='{}'); // true
 
var o = {a:1};
alert($.toJSON(o)=='{}'); // false

The code above simply converts an object to JSON string.

Your way

Well, if You have Your way of checking for an empty javascript object I am waiting Your comments! Thanks in advance!

Links

hasOwnProperty documentation

JQuery JavaScript Library

jquery-json plugin for JQuery