function myRandom() {
 var rand = Math.random();
 if (rand < .5) return 1;
 if (rand < .8) return 2;
 if (rand < .95) return 3;
 return 5;
}

function prizeRand(oArr) {
 var sum = 0; // 總和
 var rand = 0; // 每次循環產生的隨機數
 var result = 0; // 返回的對象的key

 console.log(oArr);

 // 計算總和
 for (var i in oArr) {
 sum += oArr[i][0];
 }

 // 思路就是如果設置的數落在隨機數內，則返回，否則減去本次的數
 for (var i in oArr) {
 rand = Math.floor(Math.random()*sum + 1);
 if (oArr[i][0] >= rand) {
 result = oArr[i][0];
 break;
 } else {
 sum -= oArr[i][0];
 }
 }

 return result;

}

var oArr = {'5':[5, 'Mac'], '3':[15, 'iPhone'], '2':[30, 'iPad'], '1':[50, 'iWatch']};

console.log(prizeRand(oArr));