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JavaScript: The Definitive Guide

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11.2 Multiple Windows and Explicit Window References

The difficulty with an implicit window reference is that most web browsers, including Navigator, allow more than one browser window to be open at a time. Since there can be more than one window, there must be more than one Window object, but the implicit window reference can only refer to one of them. Logically, the implicit reference is a reference to the current window--the window that is displaying the HTML document that contains the JavaScript code being executed.

If you want use the properties or methods of a Window object other than the current, implicit, window, you must obtain an explicit reference to it. In general, the only way to obtain an explicit reference to another Window object is to create that Window (and the browser window it represents) yourself. You open a new browser window, and create the Window object that represents it with the open() method of the Window object. You might use it like this. (Note that we access it through the window property of the implicit Window object to make more clear what it is we are opening.)

var newwin = window.open("sitemap.html", "site_map_window");
The first argument to this method is the URL of the document to be displayed in the new window. The second argument is a name for the new window. We'll see what this name can be used for later in this chapter. For now, note that this is not a variable name; you can't refer directly to the new window with this name. There is also a third, optional argument to the Window.open() method that specifies the size of the new window, and the features, such as a menubar, toolbar, and so on, that it should contain. See the reference section for full details on this third argument and on the method itself.

The most important feature of the open() method is the value it returns. This is the explicit reference to the new Window object that we need. In the line of code above, we store this reference in a variable named newwin. (Note the difference between the name of the variable that contains a reference to the window and the name of the window itself.) With this explicit reference to the new Window object, we can use properties and methods of the new window. For example, to set the text in the new window's status line, we could do this:

newwin.defaultStatus = "Site Map. Click map for details.";

The code shown above is intended to run in the original window, and use the newwin variable defined in that window to refer explicitly to the newly created window. Any code in the new window (i.e., JavaScript that is part of the sitemap.html document displayed in that window) can of course refer to that new window with an implicit reference--for that code, the new window is the "current" window. This raises the question of how code in the new window can refer to the original window, in order to use properties and methods of that Window object. Once again, an explicit reference is needed. In this case, the original window can provide that explicit reference for the use of the new window. The code to do so might look like this:

// Create a new window.
var newwin = window.open("sitemap.html", "site_map_window");
// Set a property in the new window that contains an explicit reference 
// to the original window. There is nothing special about the name "creator";
// we can choose any property name we want.
newwin.creator = self;
Code in the new window can use this creator property to refer back to the original window:

// Code in the new window. Note that we refer to the creator property 
// of the new window using the implicit window reference for that window.
creator.alert("Hello old window, this is the new window!");
In Navigator 3.0 and Internet Explorer 3.0, the open() method automatically creates an opener property for the new window that refers back to the window that opened it. This opener property can be used just like the creator property in the example above.

We've seen how we can use the Window.open() method to create a new browser window and obtain an explicit reference to it. The open() method also allows us to obtain an explicit reference to windows that already exist, if we know the name of that window. We mean here the name of the window itself, of course, not the name of a variable that refers to the window. This is the name specified by the second argument to Window.open(). In the examples above, we've used the name "site_map_window". So, if we know that a window by this name already exists, but we do not have a variable or a property that refers to the Window object for that window, then we can obtain such a reference like this:

// Return a reference to a named window that already exists, or, if it
// doesn't actually exist, then create a window with this name.
site_map = window.open("", "site_map_window");
The syntax used here is exactly the same as that we used when creating a window--if you specify the name of window that already exists, the open() method returns a reference to that window rather than creating a new one. On the other hand, if no window with the specified name exists, then open() creates one and returns a reference to it. Note that in Navigator 3.0 the open() sets the opener property of the named window whenever it is called, not only when it is created. So, this property of a window refers either to the window that created it or to the window that most recently looked it up by name.

Closing Windows

After all this talk of opening new windows, we should note that the Window object also has a close() method. If your program has created and used a new browser window, and that window is no longer needed, then it can close the new window with code like this:

window.close(site_map);
Or, the new window could close itself when it is no longer needed:

window.close(self);
Once a window has been closed, you should no longer use any of its properties or methods. (In Navigator 3.0, you may safely test the closed property of a closed window--if this property is true it lets you know that the window has already been closed and that you should not use any of the other properties.)

Note that you are only allowed to automatically close windows that your code created. If you attempt to close a window that the user opened, your attempt will either fail (in Navigator 2.0) or will pop up a prompt dialog asking the user if the window should really be closed (Navigator 3.0). This prevents malicious coders from creating web pages to lure unsuspecting surfers in and then close their main (and only) browser window!


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