JavaScript check if is Undefined
To check if the variable is undefined, do this:
<script>
if (typeof variable === "undefined") {
// do something
}
</script>
Example:
<div id="name"></div>
<script>
if (typeof username === "undefined") {
document.getElementById('name').innerHTML = 'Name is undefined';
} else {
document.getElementById('name').innerHTML = name;
}
</script>
Output: Name is undefined