Find length (size) of an array in JavaScript

Question

I have a simple 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

• Cameron: What are you trying to accomplish? What do you expect the result to be?

Answer 1

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).

Answer 2

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

Answer 3

Integer has no method length. Try string

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

Answer 4

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);

Answer 5

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

• Wai Ha Lee: "Why does the below return 'undefined'?"

Answer 6

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.

• tom van green: Please don't just post code without any explanation. Anyway, this code does not seem to have anything to do with what was beeing asked in the first place.

Answer 7

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.