var a;
a = 100;
console.log(a);
const b = 5; //常量必须要有初始值
console.log(b);
//查看数据类型
console.log(typeof 1000); //number 非Number 因为 number 属于 Number这个大类。
var name = '大白';
console.log(typeof name);
console.log(Number.MAX_VALUE);//最大值
console.log(Number.MAX_VALUE + Number.MAX_VALUE);
console.log(typeof Infinity);//无穷大
console.log(typeof -Infinity);//无穷小
console.log(typeof NaN);//not a number 但NaN仍然为一个数
//整数运算精确，浮点型运算会丢失精度
console.log(100 + 20); //输出120
console.log(100.1 + 20.1); //输出120.199999999999999 后期可以借助JS的类库和某些转换使精度不丢失
console.log(name);
console.log(typeof name);
var str1 = "name'chao'";//只能不同引号间进行嵌套
console.log(str1);
//变量加上引号，变量变常量
console.log('name');
//布尔型Boolean
var bool1 = Boolean(0);//fasle NaN 也是false
console.log(bool1);
var bool2 = Boolean(1); //正负值均为true
console.log(bool2);
var bool3 = Boolean(-1);
console.log(bool3);
var bool4 = Boolean("hello!"); //字符串也为true
console.log(bool4);
var bool5 = Boolean(""); //空字符串，null,undefined 也是false
console.log(bool5);
var bool6 = Boolean(null);
console.log(bool6);
var bool7 = Boolean(undefined);
console.log(bool7);
var str2 = undefined; //输出均为undefiend
var str3;
console.log(str2,str3);
console.log(typeof str2);
var str4 = null;
console.log(typeof str4);//输出object 表示一个空对象
