我听不懂这部分：“（（obj [val] || 0）+1;”

Question

let arr = [1, 2, 3, 2, 3],
  obj = {};

for (let val of arr) {
  obj[val] = (obj[val] || 0) + 1;
}

console.log(obj);

输出应为：

obj = {
  1: 1,
  2: 2,
  3: 3
}