As a rule of thumb, which of these methods of writing cross-browser Javascript functions will perform better?

Method 1

function MyFunction() 
{
    if (document.browserSpecificProperty)
       doSomethingWith(document.browserSpecificProperty);
    else
       doSomethingWith(document.someOtherProperty);
}

Method 2

var MyFunction;
if(document.browserSpecificProperty) {
    MyFunction = function() {
       doSomethingWith(document.browserSpecificProperty);
    };
} else {
    MyFunction = function() {
       doSomethingWith(document.someOtherProperty);
    };
}

Edit: Upvote for all the fine answers so far. I've fixed the function to a more correct syntax.

Couple of points about the answers so far - whilst in the majority of cases it is a fairly pointless performance enhancement, there are a few reasons one might want to still spend some time analyzing the code:

• Has to run on slow puters, mobile devices, old browsers etc.
• Curiosity
• Use the same general principal to performance enhance other scenarios where the evaluation of the IF statement does take some time.

As a rule of thumb, which of these methods of writing cross-browser Javascript functions will perform better?

Method 1

function MyFunction() 
{
    if (document.browserSpecificProperty)
       doSomethingWith(document.browserSpecificProperty);
    else
       doSomethingWith(document.someOtherProperty);
}

Method 2

var MyFunction;
if(document.browserSpecificProperty) {
    MyFunction = function() {
       doSomethingWith(document.browserSpecificProperty);
    };
} else {
    MyFunction = function() {
       doSomethingWith(document.someOtherProperty);
    };
}

Edit: Upvote for all the fine answers so far. I've fixed the function to a more correct syntax.

Couple of points about the answers so far - whilst in the majority of cases it is a fairly pointless performance enhancement, there are a few reasons one might want to still spend some time analyzing the code:

• Has to run on slow puters, mobile devices, old browsers etc.
• Curiosity
• Use the same general principal to performance enhance other scenarios where the evaluation of the IF statement does take some time.

• javascript
• performance
• cross-browser

• Alas this isn't actually cross-browser code as it relies on behaviour that only occurs in mozilla. – olliej Commented Mar 12, 2009 at 3:38

7 Answers 7

Unless you're doing this a trillion times, it doesn't matter. Go with the one that is more readable and maintainable to you and/or your organization. The productivity gains you will get from writing clean, simple code matters way more than shaving a tenth of a microsecond off your JS execution time.

You should only even start thinking about what performs better when and only when you've written code and it is unacceptably slow. Then you should start tracking down the bottleneck, which will never be something like this. You will never get a measurable performance gain out of switching from one to the other here.

Unfortunately the code above is not actually cross-browser friendly as it relies on a mozilla quirk not present in other browsers -- namely that function statements are treated as function expressions inside branches. On browsers other that aren't built on mozilla the above code will always use the second function definition. I made a simple testcase to demonstrate this here.

Basically the ECMAScript spec says that function statements are treated similarly to var declarations, eg. they all get hoisted to the top of the current execution scope (eg. the start of a <script> tag, the start of a function, or the start of an eval block).

To clarify olliej's answer, your second method is technically a syntax error. You could rewrite it this way:

var MyFunction;
if(document.browserSpecificProperty) {
    MyFunction = function() {
       doSomethingWith(document.browserSpecificProperty);
    };
} else {
    MyFunction = function() {
       doSomethingWith(document.someOtherProperty);
    };
}

Which is at least correct syntax, but note that MyFunction would only be available in the scope in which that occurs. (Omit var MyFunction;, and preferably use window.MyFunction = function() ... for global.)

Technically, I would say that the second one would perform better, because the if statement is only executed once, rather than every time the function is run.

The difference, however, would be negligible to the point of being meaningless. The performance penalty of a single if statement such as this would be insignificant even pared to the performance penalty of simply calling a function. It would make a smallish difference even if if is called a million times.

The first one is easier to understand, because it doesn't have the awkwardness of defining the same function twice based on a condition, with both versions behaving differently. That seems to be a recipe for confusion later on.

I wouldn't be the first person to say that unless you are really insane about this optimization thing, you'll get more of a win out of code readability.

I generally prefer the second version, as the condition only has to be evaluated once and not on every call, but there are times when it's not really feasible because it will hamper readability.

Btw, this is a case where you might want to use the ?: operator, e.g (taken from production code):

var addEvent =
    document.addEventListener ? function(type, listener) {
        document.addEventListener(type, listener, false);
    } :
    document.attachEvent ? function(type, listener) {
        document.attachEvent('on' + type, listener);
    } :
    throwError;

For your simplified example I would do what's below assuming that your browser property check only needs to be done once:

var MyFunction = (function() {

      var rightProperty = document.browserSpecificProperty || document.someOtherProperty;

      return function doSomethingWith() {
            // use the rightProperty variable in your function
      }
})();

The performance should be nearly equal!

Thing about using Frameworks like JQuery to get rid of the Browser pability problems!

If performance is your main goal, have a look at SlickSpeed! It is a page which benchmarks different JavaScript frameworks!