可选链(.?) 空值合并运算符(? ?) 空值赋值运算符(? ?=)

      const obj = {
        a: 1
      }
      console.log(obj.a)  //1
      //console.log(obj.b.c)  //报错
      console.log(obj.b?.c)   //undefined, 不报错， 可选链(.?)
      
      //空值合并运算符(??)
      const a = 0
      console.log(a ?? 1) // 0
      console.log(undefined ?? 1) // 1

      let result = 0
      let result2

      //空值赋值运算符(??=)
      result ??= 1
      console.log(result) //0

      result2 ??= 3
      console.log(result2)  //3