How to check element exists?

Question

I need to check if element ( #idname ) exists, then alert a message to user before closing current tab (or browser).

Here is my condition:

if (document.contains(document.getElementById("#idname"))) {
    window.onbeforeunload = function(evt) {
      // some code here
}

The above code works as well, but my problem is that element ( #idname ) isn't exist in the HTML in first and I will append it to my HTML after page loading. Now that condition doesn't work anymore. Is there any solution?

Answer 1

just check if you can select it:

$("#idname").length > 0

or

document.getElementById("idname") === null

You have to do the request inside your function! (See answer of Vohuman)

Answer 2

You can simply check the existence of the element in your onbeforeunload handler:

window.onbeforeunload = function(evt) {
   if ( document.getElementById("idname") ) {
      // ...
   }
}