[javascript] Check if an HTML input element is empty or has no value entered by user

Related to this question here. Can I check if an element in the DOM has a value or not? I'm trying to put it into an 'if' statement and not sure if my approach is correct. Here goes:

if (document.getElementById('customx')){
    //do something
}

Or should it be:

if (document.getElementById('customx') == ""){
    //do something
}

EDIT: By value I mean, customx is a text input box. How can I check if this field has no text entered.

var input = document.getElementById("customx");

if (input && input.value) {
    alert(1);
}
else {
    alert (0);
}

getElementById will return false if the element was not found in the DOM.

var el = document.getElementById("customx");
if (el !== null && el.value === "")
{
  //The element was found and the value is empty.
}

You want:

if (document.getElementById('customx').value === ""){
    //do something
}

The value property will give you a string value and you need to compare that against an empty string.

Your first one was basically right. This, FYI, is bad. It does an equality check between a DOM node and a string:

if (document.getElementById('customx') == ""){

DOM nodes are actually their own type of JavaScript object. Thus this comparison would never work at all since it's doing an equality comparison on two distinctly different data types.