In the past few years, performance research has become more prevalent thanks to research by the Yahoo! Exceptional Performance Team and Google’s Steve Souders. Most of that research studies not the individual HTML page, but the resources the page requires to display or behave correctly.

Although both CSS and JavaScript may be included within an HTML page, best practices encourage storing CSS and JavaScript in external files that can be downloaded and cached separately. Performance research asks: How can these external resources be downloaded and applied most efficiently? The first approach is to limit the number of external requests since the overhead of each HTTP request is high. The second approach? Make your code as small as possible.

The history of JavaScript byte savings

Douglas Crockford introduced JSMin in 2004 as a way to shrink JavaScript files before placing them into a production environment. His simple tool removed spaces, tabs, and comments from JavaScript files, effectively decreasing the size compared to the original source file. His rationale was sound: decreasing the size of JavaScript code results in faster downloads and a better user experience.

Three years later, Yahoo! engineer Julien Lecomte introduced the YUI Compressor. The YUI Compressor’s goal was to shrink JavaScript files even further than JSMin by applying smart optimizations to the source code. In addition to removing comments, spaces, and tabs, the YUI Compressor also safely removes line breaks, further decreasing the overall file size. The biggest byte savings, though, come from replacing local variable names with one- or two-character names. For example, suppose you have the following function:

function sum(num1, num2) {
    return num1 + num2;
}

YUI Compressor changes this to:

function sum(A,B){return A+B;}

Note that the two local variables, num1 and num2, were replaced by A and B, respectively. Since YUI Compressor actually parses the entire JavaScript input, it can safely replace local variable names without introducing code errors. The overall function continues to work as it did originally since the variable names are irrelevant to the functionality. On average, the YUI Compressor can compress files up to 18% more than JSMin.

These days, it’s common to use a minification tool plus HTTP compression to further reduce JavaScript file size. This results in even greater savings than using either method alone.

Boosting minification

A couple of years ago, as I started debugging large amounts of production code, I realized that the YUI Compressor didn’t apply variable replacement to a fairly significant portion of my code. Bothered by what I considered a lot of wasted bytes, I explored coding patterns to boost the YUI Compressor’s minification powers. I presented my results, Extreme JavaScript Compression with YUI Compressor, internally at Yahoo!.

In my investigation, I discovered coding patterns that prevented YUI Compressor from performing variable name replacement. By modifying or avoiding these coding patterns, you can improve the YUI Compressor’s performance.

JAVASCRIPT’S EVIL FEATURES

Anyone who has followed Douglas Crockford’s writing or lectures knows about the “evil” parts of JavaScript: The parts that are confusing and/or that prevent us from writing clean code that performs well. The eval()function and the with statement are the two most egregious examples of evil JavaScript. Though there are other considerations, both of these features force YUI Compressor to stop replacing variables. To understand why, we need to understand the intricacies of how each works.

WORKING WITH EVAL()

The eval() statement’s job is to take a string and interpret it as JavaScript code. For example:

eval("alert('Hello world!');");

The tricky part of eval() is that it has access to all of the variables and functions that exist around it. Here’s a more complex example:

var message = "Hello world!";

function doSomething() {
    eval("alert(message)");
}

When you call doSomething(), an alert is displayed with the message, “Hello world!”. That’s because the string passed into eval() accesses the global variable message and displays it. Now consider what would happen if you automatically replaced the variable name message:

var A = "Hello world!";

function doSomething() {
    eval("alert(message)"); 
}

Note that changing the variable name to A results in an error when doSomething() executes (sincemessage is undefined). YUI Compressor’s first job is to preserve the functionality of your script, and so when it sees eval(), it stops replacing variables. This might not sound like such a bad idea until you realize the full implications: Variable name replacement is prevented not only in the local context where eval() is called, but in all containing contexts as well. In the previous example, this means that both the context inside ofdoSomething() and the global context cannot have variable names replaced.

Using eval() anywhere in your code means that global variable names will never be changed. Consider the following example:

function handleJSONP(object) {
    return object;
}

function interpretJSONP(code) {
    var data = eval(code);
    
    //process data
}

In this code, pretend that handleJSONP() and interpretJSONP() are defined in the midst of other functions.JSONP is a widely used Ajax communication format that requires the response to be interpreted by the JavaScript engine. For this example, a sample JSONP response might look like this:

handleJSONP({message:"Hello world!"});

If you received this code back from the server via an XMLHttpRequest call, the next step is to evaluate it, at which point eval() becomes very useful. But just having eval() in the code means that none of the global identifiers can have their names replaced. The best option is to limit the number of global variables you introduce.

You can often get away with this by creating a self-executing anonymous function, such as:

(function() {
    function handleJSONP(object) {
        return object;
    }

    function interpretJSONP(code) {
        var data = eval(code);
    
        //process data
    }
})();

This code introduces no new global variables, but since eval() is used, none of the variable names will be replaced. The actual result (110 bytes) is:

(Line wraps marked » —Ed.)

(function(){function handleJSONP(object){return object}function »
interpretJSONP(code){var data=eval(code)}})();

The nice thing about JSONP is that it relies on the existence of just one global identifier, the function to which the result must be passed (in this case, handleJSONP()). This means that it doesn’t need access to any local variables or functions and gives you the opportunity to sequester the eval() function in its own global function. Note that you also must move handleJSONP() outside to be global as well so its name doesn’t get replaced:

//my own eval
function myEval(code) {
    return eval(code);
}

function handleJSONP(object) {
    return object;
}

(function() {
    function interpretJSONP(code) {
        var data = myEval(code);
    
        //process data
    }
})();

The function myEval() now acts like eval() except that it cannot access local variables. It can, however, access all global variables and functions. If the code being executed by eval() will never need access to local variables, then this approach is the best. By keeping the only reference to eval() outside of the anonymous function, you allow every variable name inside of that function to be replaced. Here’s the output:

function myEval(code){return eval(code)}function handleJSONP »
(a){return a}(function(){function a(b){var c=myEval(b)}})();

You can see that both interpretJSON(), code, and data were replaced (with a, b, and c, respectively). The result is 120 bytes, which you’ll note is larger than the example without eval() sequestered. That doesn’t mean the approach is faulty, it’s just that this example code is far too small to see an impact. If you were to apply this change on 100KB of JavaScript code, you would see that the resulting code is much smaller than leavingeval() in place.

Of course, the best option is not to use eval() at all, as you’ll avoid a lot of hoop-jumping to make the YUI Compressor happy. However, if you must, then sequestering the eval() function is your best bet for optimal minification.

The with statement

The with statement is the second evil feature that interferes with the YUI Compressor’s variable replacement technique. For those unfamiliar, the with statement is designed (in theory) to reduce the size of code by eliminating the need to write the same variable names over and over again. Consider the following:

var object = {
    message: "Hello, ",
    messageSuffix: ", and welcome."
};
object.message += "world" + object.messageSuffix;
alert(object.message);

The with statement allows you to rewrite this code as:

var object = {
    message: "Hello, ",
    messageSuffix: ", and welcome."
};
with (object) {
    message += "world" + messageSuffix;
    alert(message);
}

Effectively, the with statement avoids the need to repeat “object” multiple times within the code. But these savings come at a cost. First, there are performance implications from using the with statement, as local variables become slower to access. This happens because variables inside of a with statement are ambiguous until execution time: They may be properties of the with statement’s context object or they may be variables from the function or another execution context. To understand this ambiguity better, take a look at the code when the local variable message is added and the definition of object is removed:

var message = "Yo, ";

with (object) {
    message += "world" + messageSuffix;
    alert(message);
}

When the identifier message is used inside of the with statement, it could be referencing the local variablemessage or it could be referencing a property named message on object. Since JavaScript is a late binding language, there is no way to know the true reference for message without completely executing the code and determining if object has a property named message. Witness how late binding affects this code:

function displayMessage(object) {
    var message = "Yo, ";

    with (object){
        message += "world" + messageSuffix;
        alert(message);
    }
}

displayMessage({ message: "Hello, ", messageSuffix: ", and welcome." });
displayMessage({ messageSuffix: ", and welcome." });

The first time that displayMessage() is called, the object passed in has a property named message. When the with statement executes, the reference to message is mapped to the object property and so the displayed message is, “Hello, world, and welcome.” The second time, the object passed in has only themessageSuffix property, meaning the reference to message inside of the with statement refers to the local variable and the message displayed is therefore, “Yo, world, and welcome.”

Since the YUI Compressor doesn’t actually execute the JavaScript code, it has no way of knowing whether identifiers in a with statement are object properties (in which case, it is not safe to replace them) or local variable references (in which case, it is safe to replace them). The YUI Compressor treats the with statement the same as eval(): when present, it will not perform variable replacement on the function or any containing execution contexts.

Unlike eval(), there is no way to sequester the with statement in such a way that it doesn’t affect most of the code. The recommendation is to avoid using the with statement at all. Even though it appears to save bytes at the time of code writing, you actually end up losing bytes by forfeiting YUI Compressor’s variable replacement feature. The displayMessage() function gets minified like this:

function displayMessage(object){var message="Yo, ";with(object) »
{message+="world"+messageSuffix;alert(message)}};

This result is 112 bytes. If the function is rewritten to avoid the with statement, displayMessage() looks like this:

function displayMessage(object) {
    var message = "Yo, ";

    object.message += "world" + object.messageSuffix;
    alert(object.message);
}

When minified, this new version of the function becomes:

function displayMessage(a){var b="Yo, ";a.message+="world"+ »
a.messageSuffix;alert(a.message)};

The size of this result is 93 bytes, so even though the original source code is larger. The minified source code becomes smaller because we used variable replacement.

Conclusion

YUI Compressor’s variable replacement functionality can give big byte savings while minifying your JavaScript. Since the YUI Compressor tries to avoid breaking your code by incorrectly replacing variable names, it will turn off variable replacement when the eval() function or with statement is used. These “evil” features alter how JavaScript code is interpreted and prevent the YUI Compressor from safely replacing variable names, which costs you a large amount of byte savings. Avoid this penalty by steering clear of eval() or sequester it away from the rest of your code. Also, avoid the with statement. These steps will ensure that your code doesn’t get in the way of optimal minification. 

— http://www.alistapart.com/articles/better-javascript-minification/