Understanding syntax of multi dimensional array in javascript

There are no multi-dimensional arrays in JavaScript. There are only nested arrays.

[            //  test
  [          //    test[0]
    [1,2],   //      test[0][0]
    [2,3],   //      test[0][1]
    [3,4]    //      test[0][2]
  ]          //
]            //

As you can see, test[0] has a length of three.

And test[[0]] is semantically incorrect^(*) and collapses into test[0].

^(*) The index operator ([]) expects a number, like in test[0]. If you don't pass a number to it (like in your test[[0]] case, where you pass the array [0]), a conversion to string will happen first. (This is because of the first note below.)

Arrays are converted to string by joining their members with a comma. [0].toString() is "0", and therefore test[[0]] is equivalent to test["0"], which is equivalent to test[0].

Notes:

• The square brackets are used for property access as well, so that test["length"] is the same as test.length.
• Consequently, something horrible like test[[0]][["length"]]) is equivalent to test[0].length and will give you 3.
• Something like test[[0,0]] would be test["0,0"] - and since there is no property named "0,0" on that array, you will get undefined.

The array test[0] contains three items:

• [1,2]
• [2,3]
• [3,4]

Hence, the result of the length is 3.

The length of array test is just one, since test contains only one array:

• [1,2],[2,3],[3,4]

In fact this is not a multi-dimensional array, it's just an array containing arrays (called a jagged array, or nested array).

test[[0]] is same thing as test[0] and test[0] is an array

[1,2],[2,3],[3,4]

consisting of these elements.

if you want to access ,for instance, [2,3] you need to use this syntax:

test[0][1]

and test[0][1].length will give you 2.

This is a nested array. An array of arrays.

It will be more clear if you expand your example:

var test = [
    [
        [1,2],[2,3],[3,4]
    ], 
    [
        [5,6],[7,8],[9,9],[6,7]
    ], 
];
alert(test[[1]].length);

Now, your test is an array of two arrays. The first one of which is an array of 3 arrays. The second one is an array of 4 arrays.

Now, test.length is 2. test[0].length is 3. test[1].length is 4. [[n]] just collapses.

Demo: http://jsfiddle.net/abhitalks/6H7Lj/

If you want get the length of first arrays length then write

alert(test[0][0].length);