More on Crockford’s and Flanagan’s approaches to JavaScript

Brian Dillard, Tuesday, August 5, 2008 @ 1:03 pm

In my recent review of "JavaScript: The Good Parts," I compared this new Douglas Crockford treatise with David Flanagan's canonical "JavaScript: The Definitive Guide." I subsequently stumbled upon a perfect example of the authors' divergent approaches to my native programming language.

Here's Flanagan discussing the dreaded with statement:

Despite its occasional convenience, use of the with statement is frowned upon. JavaScript code that uses with is difficult to optimize and may therefore run more slowly than the equivalent code written without the with statement. Furthermore, function definitions and variable initializations within the body of a with statement can have surprising and counterintuitive behavior.* For these reasons, it is recommended that you avoid the with statement.

* This behavior, and the reasons behind it, are too complicated to explain here.

And here's Crockford:

JavaScript has a with statement that was intended to provide a shorthand when accessing the properties of an object. Unfortunately, its results can sometimes be unpredictable, so it should be avoided.

The statement:

with (obj) {
    a = b;
}

does the same thing as:

if (obj.a === undefined) {
    a = obj.b === undefined ? b : obj.b;
} else {
    obj.a = obj.b === undefined ? b : obj.b;
}

So, it is the same as one of these statements:

a = b;
a = obj.b;
obj.a = b;
obj.a = obj.b;

It is not possible to tell from reading the program which of those statements you will get. It can vary from one running of the program to the next. It can even vary while the program is running. If you can't read a program and understand what it is going to do, it is impossible to have confidence that it will correctly do what you want.

Simply by being in the language, the with statement significantly slows down JavaScript processors because it frustrates the lexical binding of variable names. It was well intentioned, but the language would be better if it didn't have it.

The difference between these approaches to the same topic can't help but remind me of a favorite episode of "Buffy the Vampire Slayer":

Oz: [to Faith] I'm wondering about your position on werewolves.

Willow: [proudly] Oz is a werewolf.

Buffy: It's a long story.

Oz: I got bit.

Buffy: Apparently not that long.

--BtVS S03E03: "Faith, Hope & Trick"

There's value in exhaustively documenting a language. But without concrete examples of why something's bad, programmers - obstinate autodidacts that they are - probably won't heed your advice. The Crockford book doesn't always provide such persuasive logic, but it usually does, making it ideal for JavaScript noobs who need to understand how to use the language safely and effectively.

agile ajax

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