JS basics--judgment

JS判断

if/else if/ else

三元运算符

简单的if else 书写,如果复杂的话,Don't write it as a ternary operator.If the statement is not executed after the judgment,则需要写null 或 undefined来占位
例：将如下if else The judgment turns into a ternary operator

let a = 10;
if (a > 0) {
    
    if (a < 10) {
    
        a++;
    } else {
    
        a--;
    }
} else {
    
    if (a > -10) {
    
        a += 2;
    }
}

由于最后一个if没有else 语句,所以需要加null 或者undefined进行赋值.转换后的结果为：

a > 0 ? (a < 10 ? a++ : a--) : (a > -10 ? a += 2 : null);

如果不加null或undefined,则报错：

Uncaught SyntaxError: Unexpected token ')'

switch case

1. 每种caseBest to add it laterbrea.不加break,It does not matter whether the latter condition holds or not,都要执行,直到遇到break为止.
   None of the conditions hold,则写default;
2. ** When a variable has several different values,The executed statement is the same as when,可以不加break;**
3. 每种caseThe equal sign in is绝对相等（‘=’包括数值类型）,和if else里面不同,if else里面是. For business rigor,使用===

例： 幂的表示： Math.pow(x,次方数)

let a = 10;
switch (a) {
    
    case 1:
        a += 1;
        break;
    case 5:
        a += 5;
        break;
    default:
        a = Math.pow(a, 2);
}
console.log(a);
//输出100

例： 如果将case1里的break去除,则为：

let a = 1;
switch (a) {
    
    case 1:
        a += 1;
    case 5:
        a += 5;
        break;
    default:
        a = Math.pow(a, 2);
}
console.log(a);

输出7

case 1 执行,case 5 虽然不符合a=1, 但是由于case1 没有break,will also be executed.

当然,break Not adding is also useful.比方说,不同的取值,When corresponding to the same execution statement,就可以不加break;
例：

let a = 1;
switch (a) {
    
    case 1:
    case 5:
        a++;
        break;
    default:
        a = Math.pow(a, 2);
}
console.log(a);
// 2

a为1或5 时,做自增操作.因为case 1 里面没有内容,且没有break,所以继续执行5里面的内容,遇到break 跳出判断.