Why TypeError: document.getElementById() is null

Question

Why the following code generates TypeError: document.getElementById("docPrint") is null

var printwindow = window.open('', '', 'fullScreen=no');
printwindow.document.write('<iframe id="docPrint" width="100%" height="100%" src="http://localhost:8080/hiring/docs/Keneth _1340800082258/Keneth _resume_1340800082258.pdf"></iframe>');
printwindow.self.focus();
document.getElementById('docPrint').focus();
document.getElementById('docPrint').contentWindow.print();

Answer 1

You are operating across two windows.

printwindow.document.write
document.getElementById

If you want to get the element you created in the popup then you have to call it's gEBI method.

printwindow.document.write
printwindow.document.getElementById

Answer 2

Prepend printwindow. to each instance of document.getElementById :

printwindow.document.getElementById('docPrint').focus();
printwindow.document.getElementById('docPrint').contentWindow.print();

Answer 3

You need to prefix your calls to document.getElementById with 'printwindow':

printwindow.document.getElementById('docPrint').focus();
printwindow.document.getElementById('docPrint').contentWindow.print();

You might also want to keep a reference to the element in a variable, to avoid the boilerplate

var el = printwindow.document.getElementById('docPrint');
el.focus();
el.contentWindow.print();