random.as

public static function generateRandomResultBasedOnWeights(weights:Array):int {
		weights.sort(Array.DESCENDING);
		var sum:Number = sumArrayElements(weights);
		var r:Number = Math.random() * sum;
		var result:int = -1;

		for (var i:uint = 0; i < weights.length; i++) {
			var item:int = weights[i];
			if (r < item)
				return result;
			r -= item;
		}
		return result;
	}

random.js

function generateRandomResultBasedOnWeights(weights) {
    //weights.sort(sortFunction);
    var sum = sumArrayElements(weights);
    console.log("sum: " + sum);
    var r = Math.random() * sum;
    var result = {value: "nothing"};

    for (var i = 0; i < weights.length; i++) {
        var item = weights[i];
        if (r < item.weight)
            return item;
        r -= item.weight;
    }
    return result;
}

function sumArrayElements(array){
    var result = 0;
    for(var i = 0; i<array.length; i++){
        result += array[i].weight;
    }
    return result;
}

function sortFunction(a, b) {
    return b.weight - a.weight;
}

function main(){
    var arr = [
        {value:"gold", weight:70},
        {value:"potion", weight:20},
        {value:"shit", weight:10}
    ];

    arr = arr.sort(sortFunction);

    var results = {
        gold: 0,
        potion: 0,
        shit: 0,
        nothing: 0
    };

    for(var i=0; i<10000; i++){
        var result = generateRandomResultBasedOnWeights(arr);
        results[result.value]++;
    }

    alert("gold " + results.gold + "\npotion " + results.potion + "\nshit " + results.shit + "\nnothing " + results.nothing);
    //output: gold ~7000, potion ~2000, shit ~1000
}

main();