let buffer = [0, 1, 2, 3, 4]; // 奖池

let buff_probability = [0.2, 0.2, 0.2, 0.2, 1]; // 概率

for (let i = 0; i < 10000; i++) {
  const result = randomProbability(buffer, buff_probability);
  console.log('result: ', result);
}

function randomProbability(elements, weightArr) {
  var sum = 0;
  var factor = 0;
  var random = Math.random();
  for (var i = weightArr.length - 1; i >= 0; i--) {
    sum += weightArr[i]; // 统计概率总和
  }
  random *= sum; // 生成概率随机数
  for (var i = weightArr.length - 1; i >= 0; i--) {
    factor += weightArr[i];
    if (random <= factor) return elements[i];
  }
  return null;
}