运算操作符
"+"
1.数学运算，字符串链接
2.任何数据类型加字符串都等于字符串"-"，"*"，"/"，"%"，"="，"()"
优先级 "=" 最弱，"()"优先级最高
"++"，"--"，"+="，"-="，"/="，"*="，"%="
运算规则
1.考虑优先级，从左到右
2."+" 任何数据类型加字符串都等于字符串
var a = 1 + "a" + 1 + 1;
console.log(a);//1a11
var a = 1 + 1 + "a" + 1;
console.log(a);//2a1
var a = 1 + 1 + "a" + 1 + (1 + 1);
console.log(a);//2a12
3."/" 特殊例子
var num = 1 / 0;
console.log(num);//Infinity(数据类型是Number)
var num = -1 / 0;
console.log(num);//-Infinity(数据类型是Number)
var num = 0 / 0;
console.log(num);//NAN(Not a Number)
4."%" 例子(取余)
var num = 1 % 5;
console.log(num);//1
var num = 7 % 5;
console.log(num);//2
var num = 5 % 5;
console.log(num);//0
5.a++ 与 ++a
++等同于+=1
var a = 1;
console.log(a++);//先打印a(此时a为1)，再a+=1(此时a为2)
//++放后面，执行完本行再+=1
var a = 1;
console.log(++a);//先a+=1(此时a为2)，再打印a(此时a为2)
//++放前面，先执行+=1再执行本行
var a = 1;
var b = a++ +1;
//先执行b = a + 1，再执行 a+=1
console.log(a);//2
console.log(b);//2
var a = 1;
var b = ++a +1;
//先执行 a+=1，再执行b = a + 1
console.log(a);//2
console.log(b);//3
var a = 1;
var b = a -- + -- a;
//先执行 a-=1，再执行b = a + a,最后a-=1
console.log(a);//-1
console.log(b);//0
6."%="例子
var a = 1;
var b = 3;
b = b % a + 3;
console.log(b)//3,a+3先算，再
