Arguments Speed Boost

When I look at code like the memoizer, I find things that others gloss over because of my training. My most recent project involves a huge application with brutal SLA’s. We must look at every function call or loop critically in order to achieve the required speed. That often means A/B testing in multiple browsers to see where we can shave a few milliseconds.

Here’s another example of what I’m talking about. How often do you see developers manipulate arguments in a function the way the memoizer did?

var args = Array.prototype.slice.call(arguments);

It’s pretty standard. Well, it may seem trivial, but nothing’s free. If you don’t have to do it, don’t–that’s the easiest way to boost performance. I wrote two example tests for the most common uses of arguments: iterating and prepending.

(function () {
    var i, len;
    //just iterate, you don't need an Array method (forEach is slow!)
    for (i = 0, len = arguments.length; i < len; i += 1) {
       noop(arguments[i], i);
    }
}(3, 2, 1));


var list = [3];
(function () {
    var i, len, llen;
    //we know getting length once is fast, so do it here then add them
    for (i = 0, llen = list.length, len = llen + arguments.length; i < len; i += 1) {
        //determine which set to use based on i and the length of the prepended set
        //just use property access to get arguments out, no slice, no concat
       console.log(llen > i ? list[i] : arguments[i - llen], i);
    }
}(2, 1));

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