gistfile1.js

Array.convert = function convert(list) {
    return Array.prototype.slice.call(list);
}

function buildCSSProperties() {
    var properties = Array.convert(document.defaultView.getComputedStyle(document.documentElement, ""));
    var length = properties.length;
    for (var i = 0; i < length; ++i) {
        var propertyWords = properties[i].split("-");
        var j = propertyWords.length;
        while (--j) {
             var shorthand = propertyWords.slice(0, j).join("-");
             if (typeof document.documentElement.style[shorthand] !== "undefined" && properties.indexOf(shorthand) < 0)
                 properties.push(shorthand);
         }
    }
    return properties.sort();
}


function buildCSSProperties2() {
    var used = {}; // hash table lookup instead of scanning the list
    var properties = []; // build the properties ourselves
    var style = document.documentElement.style; // cache the style, prevent numerous property lookups
    var list = document.defaultView.getComputedStyle(document.documentElement, "");
    var length = list.length;
    for (var i = 0; i < length; ++i) // fill the hash table early with known values
        used[properties[i] = list[i]] = true; // combine statements (1) mark as used (2) add to list

    for (var i = 0, end = length; i < length; ++i) { // keep end pointer to prevent push() calls
        var propertyWords = properties[i].split("-");
        var j = propertyWords.length;
        while (--j) {
            propertyWords.pop(); // pop() instead of slice()
            var shorthand = propertyWords.join("-");
            if (!(shorthand in used) && style[shorthand] !== undefined) { // hash lookup first, and no typeof
                used[shorthand] = true;
                properties[end++] = shorthand;
            }
        }
    }

    return properties.sort();
}

var TOTAL = 1000;
var RUN1, RUN2;

// Compare Equality
var x = buildCSSProperties();
var y = buildCSSProperties2();
for (var i=0; i<x.length; ++i)
    if (x[i] !== y[i])
        console.log('not equal');
if (x.length !== y.length)
    console.log('not equal');

// Time version 1
(function() {
    var s = Date.now();
    console.time("version 1");
    for (var i=0; i<TOTAL; ++i)
        buildCSSProperties();
    console.timeEnd("version 1");
    var e = Date.now();
    RUN1 = e-s;
})();

// Time version 2
(function() {
    var s = Date.now();
    console.time("version 2");
    for (var i=0; i<TOTAL; ++i)
        buildCSSProperties2();
    console.timeEnd("version 2");
    var e = Date.now();
    RUN2 = e-s;
})();

// Compare
console.log("verison 2 is " + (RUN1/RUN2).toFixed(2) + " times faster");