JavaScript 不常见操作运算符详解：逗号、空值合并与可选链

var foo = (12.4 / 4.13) | 0;//结果为3
var bar = ~~(12.4 / 4.13);//结果为3

var eee="eee";
alert(!!eee)

let x = 1;
x = (x++, x);
console.log(x);
// expected output: 2
x = (2, 3);
console.log(x);
// expected output: 3

var a = 10, b = 20;

function CommaTest(){
    return a++, b++, 10;
}

var c = CommaTest();

alert(a); // 返回11
alert(b); // 返回21
alert(c); // 返回10

alert(2*5, 2*4); // 输出10

alert((2*5, 2*4)); // 返回8

let str = null||undefined
let result = str??'haorooms博客'
console.log(result)//haorooms博客

const nullValue = null;
const emptyText = ""; // 空字符串，是一个假值，Boolean("") === false
const someNumber = 42;

const valA = nullValue ?? "valA 的默认值";
const valB = emptyText ?? "valB 的默认值";
const valC = someNumber ?? 0;

console.log(valA); // "valA 的默认值"
console.log(valB); // ""（空字符串虽然是假值，但不是 null 或者 undefined）
console.log(valC); // 42

const demo = {
    name: 'haorooms',
    cat: {
        name: 'haorooms cat'
    }
};
console.log(demo.dog?.name); 
// expected output: undefined
console.log(demo.what?.());
// expected output: undefined

let result = someOne.customMethod?.();

let nestedProp = obj?.['prop' + 'Name'];

let arrayItem = arr?.[42];

let potentiallyNullObj = null;
let x = 0;
let prop = potentiallyNullObj?.[x++];

 
console.log(x); // x 将不会被递增，依旧输出 0

//当在表达式中使用可选链时，如果左操作数是 null 或 undefined，表达式将不会被计算


let customer = {
  name: "haorooms",
  details: { age: 82 }
};

let customerCity = customer?.city ?? "中国";
console.log(customerCity);  // “中国”