gistfile1.js

function buildCSSProperties() {
    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();
}

WebInspector = {};
WebInspector.CSS = {};
WebInspector.CSS.properties = buildCSSProperties();

WebInspector.CSS.properties.startsWith = function startsWith(prefix)
{
    // FIXME: Use binary search.
    return this.filter(function(property) {
        return property.indexOf(prefix) === 0;
    });
}

WebInspector.CSS.properties.startsWith2 = function startsWith2(prefix)
{
    var firstIndex = this._firstIndexOfPrefix(prefix);
    if (firstIndex === -1)
        return [];

    var results = [];
    while (this[firstIndex].indexOf(prefix) === 0)
        results.push(this[firstIndex++]);
    return results;
}

WebInspector.CSS.properties._firstIndexOfPrefix = function(prefix)
{
    if (!prefix)
        return -1;

    var maxIndex = this.length - 1;
    var minIndex = 0;
    var foundIndex;

    do {
        var middleIndex = (maxIndex + minIndex) >> 1;
        if (this[middleIndex].indexOf(prefix) === 0) {
            foundIndex = middleIndex;
            break;
        }
        if (this[middleIndex] < prefix)
            minIndex = middleIndex + 1;
        else
            maxIndex = middleIndex - 1;
    } while (minIndex <= maxIndex);

    if (!foundIndex)
        return -1;

    while (foundIndex && this[foundIndex - 1].indexOf(prefix) === 0)
        foundIndex--;

    return foundIndex;
}

WebInspector.CSS.properties.firstStartsWith = function firstStartsWith(prefix)
{
    var foundIndex = this._firstIndexOfPrefix(prefix);
    return (foundIndex === -1 ? "" : this[foundIndex]);
}

var TOTAL = 10000;
var RUN1, RUN2;
var STR = "co";

// Time version 1
(function() {
    var s = Date.now();
    console.time("version 1");
    for (var i=0; i<TOTAL; ++i)
        WebInspector.CSS.properties.startsWith(STR);
    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)
        WebInspector.CSS.properties.startsWith2(STR);
    console.timeEnd("version 2");
    var e = Date.now();
    RUN2 = e-s;
})();

console.log( WebInspector.CSS.properties.startsWith(STR) );
console.log( WebInspector.CSS.properties.startsWith2(STR) );
console.log("version 2 is " + (RUN1/RUN2).toFixed(2) + " times faster");