Occasionally it becomes useful to get the absolute form path of a URL from a relative one. You might be dynamically changing links on a page, or loading scripts from the same location as scripts that have already been loaded.

Computing an absolute URL by hand is problematic: you need to deal with any <base> elements in the document and properly implement the relative path canonicalization that browsers already do.

It turns out that on standards-compliant browsers based on Gecko, WebKit or Opera, the href property of anchor elements is magical. If you assign a URL fragment to it, it comes back as a fully-qualified URL when you read it back.

Ok, that’s great, but what about IE?  Well, it turns out that IE will fully-qualify URLs returned by the href property, but only if that anchor tag was created by the parser. Using createElement('a') and setting href won’t trigger this code path.  There’s a trick we can use to work around this limitation, however.  You can force IE to use the parser to create elements by assigning innerHTML of another element.  This runs the element creation through the magic construction path that correctly sets up the attribute/property mapping.

Here’s a snippet of a function that will correctly canonicalize any URL you pass to it:

function canonicalize(url) {
    var div = document.createElement('div');
    div.innerHTML = "<a></a>";
    div.firstChild.href = url; // Ensures that the href is properly escaped
    div.innerHTML = div.innerHTML; // Run the current innerHTML back through the parser
    return div.firstChild.href;

You can use this script for other interesting purposes, like determining the base path for the current page (returns “/” for “/foo” and “/foo/” for “/foo/”). It gets the relative URL for the path “_#”, which removes any anchors, query strings or filenames from the current URL.

function getBasePath() {
    return canonicalize("_#").slice(0,-2);

The above code was tested on Safari, Chrome, Firefox 3.5 and IE6.

Read full post

My local tests started failing soon after upgrading my machine to Safari 4.0.4. Some research and pointers from the GWT folk pointed me at the root cause, a WebKit bug which is now fixed.

Kelly Norton pointed me at a quick fix:

  1. Download the latest WebKit nightly from here
  2. Save it locally
  3. Add the following environment variable to your testing .launch targets (using the path to your new, of course). Tests run from Ant or the command line will need to use the appropriate tasks or shell commands:

    Value: /Applications/path-to-your-local-webkit/
  4. Snow Leopard users will need to use 10.6 in the path above.

The proper fix should arrive in WebKit 4.0.5 at some point, but this will keep you running for now.

Jim Douglas has posted more detailed instructions on how to configure your launch targets on the GWT issue here: Issue #4220: GWT crash (Safari 4.0.4)

Read full post

We’re just about to release our DotSpots extension for Chrome and I’ve been working on integrating the CRX packaging into our build process. CRX files are basically ZIP files with an RSA signature and public key tacked on to the front of it. Generating these files requires you to use the Chrome –pack-extension argument (which in turn requires you to deploy the 100MB+ Chrome binaries to your build machine).

The existing code to pack a Chrome extension in Python is pretty dated: it will only generated the insecure CRX version 1 format that doesn’t use a cryptographic signature. There’s some Ruby code to pack a version 2 extension, but it requires a lot of dependencies that aren’t installed by default on OSX or in Fedora.

I’ve written some code in Python that uses openssl under the hood to do the grunt work. It cuts some corners by requiring you to pre-zip your files, but you’ll get better results from 7zip -9 than Python’s internal zip code anyways. Pass it three arguments: The input ZIP file, the PEM key (generated when you manually pack the extension in Chrome for the first time) and the output file.

# Cribbed from
# and

import sys
from array import *
from subprocess import *

arg0,input,key,output = sys.argv

# Sign the zip file with the private key in PEM format
signature = Popen(["openssl", "sha1", "-sign", key, input], stdout=PIPE);

# Convert the PEM key to DER (and extract the public form) for inclusion in the CRX header
derkey = Popen(["openssl", "rsa", "-pubout", "-inform", "PEM", "-outform", "DER", "-in", key], stdout=PIPE);

out=open(output, "wb");
out.write("Cr24")  # Extension file magic number
header = array("l");
header.append(2); # Version 2

print "Done."
Read full post

Javascript has three primitive types: number, string and boolean. You can quickly coerce values between the primitive types using some simple expressions.

There are a few different coersion expressions, depending on how you want to handle some of the corner cases.  I’ve automatically generated a list below:

Conversion: To Number To Number To Number To String To Boolean To Boolean
Expression: +x (+x)||0 +(x||0) ""+x !!x !!+x
null 0 0 0 "null" false false
(void 0) NaN 0 0 "undefined" false false
NaN NaN 0 0 "NaN" false false
Infinity Infinity Infinity Infinity "Infinity" true true
-Infinity -Infinity -Infinity -Infinity "-Infinity" true true
0 0 0 0 "0" false false
"0" 0 0 0 "0" true false
1 1 1 1 "1" true true
"1" 1 1 1 "1" true true
2 2 2 2 "2" true true
"2" 2 2 2 "2" true true
[] 0 0 0 "" true false
({}) NaN 0 NaN "[object Object]" true false
true 1 1 1 "true" true true
"true" NaN 0 NaN "true" true false
false 0 0 0 "false" false false
"false" NaN 0 NaN "false" true false
"" 0 0 0 "" false false
"null" NaN 0 NaN "null" true false

The above table was generated with this code (note: uses some Firefox-specific code).

<table id="results" style="border-collapse: collapse; border: 1px solid black;">
 <tr id="header">
 <tr id="header2">

function styleCell(cell) { = '1px solid black'; = '0.2em';
 return cell;

values = [
null, undefined, NaN, +Infinity, -Infinity, 0, "0", 1, "1", 2, "2",
   [], {}, true, "true", false, "false", "", "null"

coersions = [
["To Number", "+x"],
 ["To Number", "(+x)||0"],
 ["To Number", "+(x||0)"],
 ["To String", "\"\"+x"],
 ["To Boolean", "!!x"],
 ["To Boolean", "!!+x"]

var results = document.getElementById('results');
var trHeader = document.getElementById('header');
var trHeader2 = document.getElementById('header2');

for (var i = 0; i < coersions.length; i++) {
 var th = trHeader.appendChild(styleCell(document.createElement('th')));
 th.textContent = coersions[i][0]
 th = trHeader2.appendChild(styleCell(document.createElement('th')));
 th.textContent = coersions[i][1]

for (var i = 0; i < values.length; i++) {
 var tr = results.appendChild(document.createElement('tr'));
 var rowHeader = tr.appendChild(styleCell(document.createElement('th')));
 rowHeader.textContent = uneval(values[i]);

 for (var j = 0; j < coersions.length; j++) {
 var td = tr.appendChild(styleCell(document.createElement('td')));
 td.textContent = uneval(eval("(function(x) { return "+coersions[j][1]+"})")(values[i]));

Read full post

Is it time to the world to move on from RSS and to its successor, Atom? Some considerations:

Atom has an IETF standard for syndication. Atom has an IETF standard for publication. Atom was designed for modularity. Atom supports rich, well-defined activities within feeds.

RSS is effectively frozen at 2.0:

RSS is by no means a perfect format, but it is very popular and widely supported. Having a settled spec is something RSS has needed for a long time. The purpose of this work is to help it become a unchanging thing, to foster growth in the market that is developing around it, and to clear the path for innovation in new syndication formats. Therefore, the RSS spec is, for all practical purposes, frozen at version 2.0.1. We anticipate possible 2.0.2 or 2.0.3 versions, etc. only for the purpose of clarifying the specification, not for adding new features to the format. Subsequent work should happen in modules, using namespaces, and in completely new syndication formats, with new names.

It is full of legacy tags and archaic design decisions:

The purpose of the <textInput> element is something of a mystery. You can use it to specify a search engine box. Or to allow a reader to provide feedback. Most aggregators ignore it.

We are spending all this time duplicating effort. Every feed reader needs to deal with Atom and RSS. Every blog provides an Atom feed and an RSS feed. Users trying to subscribe to blog feeds are presented with an unnecessary choice.

RSS solved a need at the time, even though it was crufty and difficult to use and difficult to parse (remember when RSS XML didn’t have to be well-formed XML?). It served as an inspiration for millions of sites to open up their content to new methods of reading. It inspired a great successor, Atom, which has surpassed it many times over.

We dropped gopher when its time ran out. It’s time to make Atom the primary format for blogs.

Read full post