JavaScript window.open 有时会返回 null

Question

I am attempting maintenance on a system I did not write (and aren't we all?). It is written in C Sharp and JavaScript, with Telerik reports.

It has the following code included in JavaScript that runs when the user clicks a button to display a report in a separate window:

var oIframe = $("iframe id='idReportFrame' style='display:none' name='idReportFrame' src=''>");
oIframe.load(function() { parent.ViewReports(); });

oIframe.appendTo('body');

try
{
  $('#idReportForm').attr('target', 'idReportFrame');
  $('#idReportForm').submit();
}
catch (err) { // I did NOT write this
            }

Then the load function:

function ViewReports()
{
  var rptName = $("#ReportNameField").val();

  if (rptName == '') { return false; }

  var winOption = "fullscreen=no,height=" + $(window).height() + "left=0,directories=yes,titlebar=yes,toolbar=yes,location=yes,status=no,menubar=yes,scrollbars=no,resizable=no, top=0, width=" + $(window).width();
  var win = window.open('@Url.Action("ReportView", "MyController")?pReportName=' + rptNameCode, 'Report', winOption);
  win.focus();
  return false;
}

When I execute this (in Chrome, at least), it does pop up the window and put the report in it. However, breakpoints in the c# code indicate that it is getting called 2 or 3 times. Breakpoints in the JavaScript and examination of the little log in the JavaScript debugging environment in Chrome show that the call to win.focus() fails once or twice before succeeding. It returns an undefined value, and then it appears that the first routine above is executed again.

I am inclined to think it some kind of timing issue, except that the window.open() call is supposed to be synchronous as far as I can tell, and I don't know why it would succeed sometimes and not others. There is a routine that gets executed on load of the window, perhaps that's somehow screwing up the return of the value from open().

I am not a JavaScript person much, as those of you that are can likely tell by this time. If there is something with the code I've put here that you can tell me is incorrect, that's great; what I'm more hopeful for is someone who can explain how the popup-report-in-frame is supposed to work. Hopefully I can do it without having to replace too much of the code I've got, as it is brittle and was not, shall we say, written with refactoring in mind.

Answer 1

From what I could find the window.open will return null when it fails to open. Something may be keeping the browser from opening additional windows a couple of times; maybe it is a popup blocker.

The actual loading of the url and creation of the window are done asynchronously.

https://developer.mozilla.org/en-US/docs/Web/API/Window.open

Answer 2

Popup blocking

In the past, evil sites abused popups a lot. A bad page could open tons of popup windows with ads. So now most browsers try to block popups and protect the user.

Most browsers block popups if they are called outside of user-triggered event handlers like onclick.

For example:

 // popup blocked window.open('https://javascript.info'); // popup allowed button.onclick = () => { window.open('https://javascript.info'); };

Source: https://javascript.info/popup-windows

Answer 3

I just ran into this and it seems to be because I had a breakpoint on the line that calls window.open and was stepping through the code, in Chrome dev tools. This was extremely hit-and-miss and seemed to fail (return null, not open a window, whether one already existed or not) more times that it succeeded.

I read @Joshua's comment that the creation is done asynchronously, so I figured that forcing the code to 'stop' each time I step might be screwing things up somehow (though on a single line like var w = window.open(...) doesn't seem like this could happen).

So, I took out my breakpoint.. and everything started working perfectly!

I also took note of https://developer.mozilla.org/en-US/docs/Web/API/Window/open where they specify that if you are re-using a window variable and name (the second argumen to window.open ) then a certain pattern of code is recommended. In my case, I am wanting to write HTML content to it, rather than give it a URL and let it async load the content over the network, and I may call the whole function repeatedly without regard for the user closing the window that pops up. So now I have something like this:

var win; // initialises to undefined
function openWindow() {
    var head = '<html><head>...blahblah..</head>';
    var content = '<h1>Amazing content<h1><p>Isn\'t it, though?</p>';
    var footer = '</body></html>';

    if (!win || win.closed) {
        // window either never opened, or was open and has been closed.
        win = window.open('about:blank', 'MyWindowName', 'width=100,height=100');
        win.document.write(head + content + footer);
    } else {
        // window still exists from last time and has not been closed.
        win.document.body.innerHTML = content;
    }
}

I'm not convinced the write call should be given the full <html> header but this seems to work 100% for me.

[edit] I found that a Code Snippet on Stackoverflow has a some kind of security feature that prevents window.open , but this jsfiddle shows the code above working, with a tweak to show an incrementing counter to prove the content update is working as intended. https://jsfiddle.net/neekfenwick/h8em5kn6/3/

Answer 4

A bilt late but I think it's due to the window not beeing actually closed in js or maybe the memory pointer not being dereferenced. I was having the same problem and I solved it by enclosing the call in a try finally block.

   try {
    if (!winRef || winRef.closed) {
        winRef = window.open('', '', 'left=0,top=0,width=300,height=400,toolbar=0,scrollbars=0,status=0,dir=ltr');
    } else {
        winRef.focus();
    }

    winRef.document.open();
    winRef.document.write(`
        <html>
            <head>
                <link rel="stylesheet" href="/lib/bootstrap/dist/css/bootstrap.min.css">
            </head>
            <body>
                ${$(id).remove('.print-exclude').html()}
            </body>
        </html>
    `);


    winRef.document.close();
    winRef.focus();
    winRef.print();
} catch { }
finally {
    if (winRef && !winRef.closed) winRef.close();
}