I'm a Golden Plover!

Today's update to the Rough Cut is particularly momentous as it features the cover of my book for the first time: the mysterious cover-choosing-cabal over at O'Reilly has chosen a golden plover as my animal, and I could not be more excited! (Contrary to popular belief, no, you do not get to pick your own animal.)

Although the Rhino has long been an icon of JavaScript books from O'Reilly, some of the more recent releases appear to be birds, so I'm thrilled to be part of that group.

The latest installment to the Rough Cut are the chapters on Client-Server Communication (20pp) and User Interface Components (50pp), which are core modules to the Closure Library, so I'm sure that this new information will be of great use to Closure neophytes (and likely some veterans, as well.)

Over the weekend, I finished drafting the chapter on Working with the Compiler Source Code. I am quite happy with the code samples that I came up with, and I hope they encourage JavaScript developers to start thinking about more innovative uses of the Compiler. Also, guest author Julie Parent has been hard at work on a chapter on goog.editor, the rich text editor widget in the Closure Library that seems to be a popular topic in the discussion group. Hopefully both of those chapters will also hit the Rough Cut in the next few weeks.

This means that 12 of the 16 chapters have been drafted, and the topics for the remaining four chapters are not as complex as those of the existing chapters, so the end is in sight! My current estimate for the final page count for the book is between 450-500 pages, which is nearly twice as long as I originally anticipated. I'm doing my best to put the Definitive in Closure: The Definitive Guide!

toString() Might Not Return a String

I was reading NCZ's JavaScript quiz, and it occurred to me that toString() is not guaranteed to return a string because it is, after all, just a method. Try to guess what is printed to the console when the following JavaScript is executed:

var obj = {
  nine: 9,
  toString: function() {
    return this.nine;
  },
  valueOf: function() {
    return 10;
  }
};

console.log(obj.toString() === '9');
console.log('' + obj === '9');
console.log('' + obj.toString() === '9');
console.log(obj + obj);
console.log(String(obj) === '9');

Because obj.toString() returns a number, not a string, obj.toString() is the number 9, not the string '9', so the first expression is false.

In the second example, I would expect that because '' is a string, the value on the right of the + would also be coerced to a string via its toString() method, which means the result would be '9'. But I am wrong! It turns out that valueOf takes precedence, so '' + obj evaluates to '10', which means '' + obj === '9' is false. It would be true if there were no valueOf method defined on obj, though.

Because toString() is called explicitly in the third example, '' + obj.toString() becomes '' + 9, which evaluates to '9', so the expression is true.

The fourth one is interesting because '99', 18, and 20 are all reasonable guesses, but it turns out that valueOf() takes precedence again, so this evaluates to 20. If there were no valueOf() method defined on obj, then the result would be 18.

In the final example, String() is used as a function (and not as a constructor function, which you should never do!), and it does, in fact, return a string, so String(obj) === '9' evaluates to true. I always assumed that the implementation of String was something like:

function(s) {
  return '' + s.toString();
};

Though a few years back, I discovered that alert.toString() throws an exception on IE6 (also alert.toString evaluates to undefined on IE6), but '' + alert and String(alert) return 'function alert() { [native code] }', as expected. At least in that case, my proposed implementation for String() would not work, though IE6 has been known to be nonstandard and buggy...

Update to Closure: The Definitive Guide

Today I'm excited to announce a significant update to Closure: The Definitive Guide. The book contains complete drafts of two new chapters that focus on the Closure Compiler: Using the Compiler and Advanced Compilation. These two chapters contribute 80 pages of new content in addition to the 167 pages that are already available.

Because I was curious, I also created a graph of my progress on the book that is automatically updated every six hours:



When I put together the original proposal for the book, I projected that it would only be 250 pages long. Clearly, that was an underestimate. This presents quite the opportunity for you, the reader, since I'd argue that the book is now underpriced as you can preorder the print version for only $25.99. For only $14 more, you can also get online and PDF access to the book today. If you do, I strongly encourage you to provide feedback on O'Reilly's web site. Remember, the book is still in draft form, so if you have suggestions on how the book could be improved, make them now so they can be included in the final version!