++的前置与后置

let a1 = 1
let b1 = 2
let c1 = b1 + a1++
console.log(a1, b1, c1);//2,2,3

let a2 = 1
let b2 = 2
let c2 = b2 + ++a2
console.log(a2, b2, c2);//2,2,4

说明：

  • 后置是先计算再++
  • 前置是先++再计算

短路运算

let a = 0;
let b = 1;
let c = a || b;
console.log(c);//1

利用这个原理，我们可以简化一些函数

let sex = prompt('您的性别?')
    if (!sex){
        sex="保密"
    }
    console.log(sex);

=>

let sex = prompt('您的性别?') || "保密"
console.log(sex);

再比如:

/*function judge(num) {
    if (num % 2 === 0) {
        return "偶数"
    } else {
        return "奇数"
    }
}*/
function judge(num) {
    return num % 2 === 0 && "偶数" || "奇数"
    //return num % 2 === 0 ? "偶数" : "奇数"
}

console.log(judge(2));//偶数

说明： 可以用来简化if条件判断语句，具体如下面这段伪代码

if (flag) {
    //...do something 
} else {
    //...do something 
}

简化为 =>

flag && {...do someting} || {...do someting}