?? js 語法糖

leetcode 2619

遇到了 ?? 語法糖，簡而言之

1. 可選鏈功能
   就是防止拋出異常

caught TypeError: Cannot read properties of undefined (reading 'salary')
    at <anonymous>:1:12

const user = {name:'11',age:'22'}
user.write.salary() == 拋出錯誤
user.write?.salary() == 顯示undefined

2. 空值凝聚
   const value1 = 0 || 'default string';
   console.log(value1);

const value2 = '' || 1000;
console.log(value2);

const value1 = 0 ?? 'default string';
console.log(value1);

const value2 = '' ?? 1000;
console.log(value2);

3. 報錯凝聚？
   return this[this.length - 1] ?? -1
   如果this.length == 0 的話原先會拋出錯誤現在不會了