if input is null javascript code example

Example 1: if input value is null do something

if(document.getElementById("question").value.length == 0)
{
    alert("empty")
}

Example 2: hhow to check input is null in html using js

function required(inputtx) 
   {
     if (inputtx.value.length == 0)
      { 
         alert("message");  	
         return false; 
      }  	
      return true; 
    }