How to find out if value is undefined or not

Question

Hi Everyone

I have this javascript in a classic asp page...

I need to find out if x.name is undefined or has a value:

x=document.getElementsByTagName("meta")[i];
if (String(x.name) != "undefined" && String(x.name) != ""){
document.write(""+x.name +": "+x.content+"<br><br>");
}

I am not sure why it is throwing an error:

document.write(""+x.name +": "+x.content+"<br><br>");

if x.name or x.content is false?

Thanks

Answer 1

It's throwing an error because you are in all likelihood trying to write the value of an element before it has been instantiated. Until your page loads none of its elements exist or and henceforth they cannot be referenced.

Answer 2

To check for undefined:

if (typeof x !== 'undefined') {
  if (x.name) {
    document.write(x.name + ': ' + x.content + '<br><br>');
  }
}

Now, I don't know what you're trying to do with that document.write() call, so that might cause problems too, but the above will make sure you only do it when "x" exists and has a non-empty "name" attribute.