Question

I have multiple div elements with same id=’mydiv’.I want to calculate these divs and Iam using the code

document.getElementById('mydiv').length

But it is not working

Answer 1

It won’t work as getElementById will always return an element with the specified ID and null if the specified ID doesn’t exist

From ECMA

 getElementById(elementId) This method returns a Element. The elementId
    parameter is of type DOMString.

What you can do is to assign each div with class

<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>
<div class="mydiv"></div>​​​​​​​​​

And iterate over:

var divs = document.getElementsByTagName('div');
var count = 0;
for(var i = 0; i < divs.length; i++) {
    if(divs[i].className == 'mydiv') count++;
}

alert(count);

If your clients support document.getElementsByClassName(), it’s even more concise:

alert(document.getElementsByClassName('mydiv').length)