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<>？？：空值合并操作符
逻辑操作符，左侧为null和undefined时，才返回右侧的数 const sum = null ?? 12 console.log(sum);
//输出12 const sum1 = 12 ?? 23 console.log(sum1); //输出12 const sum2 = undefined ??
12 console.log(sum2); //输出12
<>?. :可选链操作符
可以读取位于连接对象链深处属性的值，不必明确验证链中的每个引用是否有效
功能类似于“.” 链式操作符，不同之处在于，在引用为空null 或者 undefined 的情况下不会引起错误，该表达式短路返回值是 undefined
与函数调用一起使用时，如果给定的函数不存在，则返回 undefined。
const fa = { name: 'lming', son: { name: 'lqq' } }; const duc = fa.duc?.name;
console.log(duc); //输出undefined
<>使用：
<>1.获取一个对象更深层次的属性，即obj中的first属性下的second属性。
为了避免报错， 获取之前要判断first属性是否为null或者undefined，在获取second属性
使用“与”运算符
let num = obj.first && obj.first.second;
使用?.可选链操作符
let num = obj.first?.second; <>可选链与函数调用
调用一个可能不存在的方法时，如果被调用的方法不存在，使用可选链可以使表达式自动返回undefined而不是抛出一个异常
let result = someInterface.customMethod?.();
注：如果存在一个属性名且不是函数, 使用 ?. 仍然会产生一个 TypeError 异常 (x.y is not a function).
<>使用空值合并操作符 let customer = { name: "Carl", details: { age: 82 } }; let
customerCity= customer?.city ?? "暗之城"; console.log(customerCity); // “暗之城” <>
短路计算 let a = null; let x = 0; let prop = a?.[x++]; console.log(x); // x
将不会被递增，依旧输出 0
