Find length (size) of an array in JavaScript

I think I'm going crazy. I have a simply question that I'm struggling with for some reason.

Why does the below return 'undefined'?

var testvar={};
testvar[1]=2;
testvar[2]=3;
alert(testvar.length);

edit I originally typed testvar[1].length. I knew this to be an error. I meant testvar.length

Because 2 isn't an array, it's a number. Numbers have no length.

Perhaps you meant to write testvar.length; this is also undefined, since objects (created using the { ... } notation) do not have a length.

Only arrays have a length property:

var testvar = [  ];
testvar[1] = 2;
testvar[2] = 3;
alert(testvar.length);    // 3

Note that Javascript arrays are indexed starting at 0 and are not necessarily sparse (hence why the result is 3 and not 2 -- see this answer for an explanation of when the array will be sparse and when it won't).

testvar[1] is the value of that array index, which is the number 2. Numbers don't have a length property, and you're checking for 2.length which is undefined. If you want the length of the array just check testvar.length

Integer has no method length. Try string

var testvar={};
testvar[1]="2";
alert(testvar[1].length);

If length is undefined you can use:

function count(array){

	var c = 0;
	for(i in array) // in returns key, not object
		if(array[i] != undefined)
			c++;

	return c;
}

var total = count(array);

       var mode = [];
                $("input[name='mode[]']:checked").each(function(i) {
                    mode.push($(this).val());
                })
 if(mode.length == 0)
                {
                   alert('Please select mode!')
                };

obj={};

$.each(obj, function (key, value) {
    console.log(key+ ' : ' + value); //push the object value
});

for (var i in obj) {
    nameList += "" + obj[i] + "";//display the object value
}

$("id/class").html($(nameList).length);//display the length of object.

var array=[];

array.push(array);  //insert the array value using push methods.
    
for (var i = 0; i < array.length; i++) {    
    nameList += "" + array[i] + "";          //display the array value.                                                             
}

$("id/class").html(array.length);   //find the array length.