javascript抽奖插件+概率计算

function Probability(conf) {
    this.probArr = conf || [];
    this.range = [],
    this.len = this.probArr.length;
    if (this.len > 0) { 
        this.init();
    }
}
Probability.prototype = {
    init: function() {
        this.setRange();
    },
    get: function() { 
        var len = this.len,
            range = this.range,
            last,
            randNum, 
            i = 0; 
        if (len === 0) {
            return;
        } else if(len === 1) {
            return 0;
        } 
        last = range[len -1];
        randNum = Math.floor(last* Math.random()); 
        for (; i < len; i++) { 
            if (randNum < range[i]) {

                break;
            }
        } 
        return i;
    }, 
    setRange: function() {
        var range = [],
            probArr = this.probArr,
            i = 0,
            len = probArr.length; 
        for(; i<len; i++) {
            var now = probArr[i],
                last = range[i-1] || 0; 
            range.push(now+last);
        }

        this.range = range;
    }
};