How to block users from closing a window in Javascript?

What will you do when a user hits ALT + F4 or closes it from Task Manager

Why don't you keep track if they did not complete it in a cookie or the DB and when they visit next time just bring the same screen back...:BTW..you haven't finished filling this form out..."

Of course if you were around before the dotcom bust you would remember porn storms, where if you closed 1 window 15 others would open..so yes there is code that will detect a window closing but if you hit ALT + F4 twice it will close the child and the parent (if it was a popup)

Take a look at onBeforeUnload.

It wont force someone to stay but it will prompt them asking them whether they really want to leave, which is probably the best cross browser solution you can manage. (Similar to this site if you attempt to leave mid-answer.)

<script language="JavaScript">
    window.onbeforeunload = confirmExit;
    function confirmExit() {
        return "You have attempted to leave this page. Are you sure?";
    }
</script>

Edit: Most browsers no longer allow a custom message for onbeforeunload.

See this bug report from the 18th of February, 2016.

onbeforeunload dialogs are used for two things on the Modern Web:

1. Preventing users from inadvertently losing data.
2. Scamming users.

In an attempt to restrict their use for the latter while not stopping the former, we are going to not display the string provided by the webpage. Instead, we are going to use a generic string.

Firefox already does this[...]

If you don't want to display popup for all event you can add conditions like

window.onbeforeunload = confirmExit;
    function confirmExit() {
        if (isAnyTaskInProgress) {
           return "Some task is in progress. Are you sure, you want to close?";
        }
    }

This works fine for me