How do you check if an input box value is undefined?

I have an input box that exists yet its value is undefined how can I check whether it is undefined so that I can execute some error code?

 

Try

if(document.getElementById("ELEMENT_ID").value){
// your code
}

Code (markup):

 

this is better exmaple:
if(document.getElementById("ElementID")){
// your code goes here
}

 

pixelz,

but Imozeb wants to check its value is undefined or not. You suggest to check the element, not its value.

Regards