Category: blog

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Using Javascript To Forward A User (And Keeping Track Of The Forward)

URL hashes (the part after the # sign) are useful for keeping track of application state. Here’s a short example of how they can be used.

The below code fragment forwards the user to a Google search for the value held in the variable q . The useful part is that this code also sets a hash to mark that the forward has taken place. This allows the page to keep track of the forwarding status, and to prevent reissuing the forward – for instance, if the user presses the back button after being forwarded.

if (location.hash != "#forwarder") {
    location.hash = "forwarder";
    var search_url = "http://www.google.com/search?q=" + escape(q);
    //Set the timeout to 10 seconds.
    setTimeout(function(){location.assign(search_url);},10000);
}

To see how this works, consider the below example. The first alert will show that there’s no hash recorded in the URL (the alert will be blank). The second line sets a hash of forwarder and the third line shows the value of the current hash (which will be the value we set on the second line). This will cause the if statement to evaluate to false, preventing the forwarding from taking place. The same process (recognizing the hash and blocking the forward) happens when the user clicks the back button on the above forwarding code.

alert(location.hash);
location.hash = "forwarder";
alert(location.hash);
if (location.hash != "#forwarder") {
    location.hash = "forwarder";
    var search_url = "http://www.google.com/search?q=" + escape(q);
    //Set the timeout to 10 seconds.
    setTimeout(function(){location.assign(search_url);},10000);
}
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The Term (Domain) Is Not Allowed

While configuring a domain within Google Apps, you may see the error message “ The Term [Domain Name] Is Not Allowed ”:

This error can come up due to several problems. The most likely issue is that the domain has been configured as an independent domain, rather than as an alias domain. A quick fix to this issue is to delete this domain from Google Apps, and then to re-add it. When the prompt comes up to select independent domain versus alias domain, select the alias domain option.

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Slashdot Downtime Notices

I like to collect samples of downtime notices and error messages from popular websites; it’s always fascinating to see how sites handle messaging on technical issues.

Here’s an example downtime notice from Slashdot:

Here is the message in context:

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Retrieving URL Parameter Values With Pure Javascript

Here is a short but incredibly useful piece of Javascript: it retrieves the value of a named URL parameter. For example, if the current page’s address is index.html?example=true&name=vinny this function would return vinny if passed the name name , and it would return true if passed the name example .

function getParameterByName(name) {
    var match = RegExp('[?&]' + name + '=([^&]*)').exec(window.location.search);
    return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
}

A note of warning: calling this method repeatedly is inefficient, since the regular expression needs to be repeated every time.

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Webmaster Tool Has Not Been Enabled

While configuring a new Google Apps account for a customer, I encountered the error below:

Here’s how to fix this. First, go to the Users tab, then click Services :

Type in Webmaster Tools in the search box directly under the Services tab:

Click the On button next to the Webmaster Tools label:

Remember to save your changes:

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JSON, Javascript, and JSONP MIME Types

A quick note about MIME types: JSON responses have a MIME type of application/json while Javascript files use application/javascript . JSONP is a JSON object within a Javascript function call, so it shares the same MIME type as Javascript ( application/javascript ).

With that said, there are some browsers (largely older browsers) and applications that don’t understand the application/json MIME type. They may require JSON responses to have a content type of application/javascript , application/x-javascript , or text/html . If you encounter issues with handling JSON, it’s a good idea to try changing the MIME type – it may solve the problem.

As a reminder, here’s how to set the content type of a response in Java (other languages have similar functions):

resp.setContentType(mime_type);

The resp object represents a javax.servlet.http.HttpServletResponse reference.

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Google Services Modifying Site Navigation

A quick note about some Google UI changes: Google is in the middle of removing the top black navigation bar.

Previously, Google had a top nav bar linking to various Google services:

Now that navigation bar is gone:

Google is popping up a little note to explain the changes:

In short, the grid icon drops down a list of various Google services to choose from:

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Generating Callbacks For JSONP Code

Here’s a code snippet demonstrating how to wrap JSON text within a JSONP response.

The text of the JSON object is in the string json. The variable resp represents a HttpServletResponseobject. Callback represents contents of the HTTP parameter callback , which should be set with the name of the Javascript function to call. Before calling this code, it’s a good idea to validate that parameter.

String callback = req.getParameter("callback");
String json_text = callback + "(" + json + ");";
resp.getWriter().println(json_text);

Here’s an example of a simple JSONP response:

findIP({"ip": "8.8.8.8"});

This response will call the JS function findIP (the callback parameter contents), with an object containing the property ip (the JSON data).

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Java GET Using java.net

Here’s a simple code snippet showing how to fetch the contents of a URL. The URL to fetch is in the string url_string , and html stores the contents of the fetched file.

URL url = new URL(url_string);
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream()));
String html = "";
String line = "";
while ((line = reader.readLine()) != null) {
    html += line;
}
reader.close();

This code may throw a java.io.IOException if an error is encountered while fetching the URL.

This code requires the below imports:

import java.net.URL;
import java.io.InputStreamReader;
import java.io.BufferedReader;