声明提前
console.log(a);
var a=3;
var a;
console.log(a);
a=3;
总结：
number+string拼接
number+boolean
boolean转number
nuber+undefined NAN
boolean+boolean求和
隐式转换结论
1.number + number 求和
var num3=num1+num2;
console.log(num3);
console.log(typeof num3);
2.number+string 拼接
var str=num1+str1;
console.log(str);
console.log(typeof str);
3.number+boolean boolean 转换为 number
var boo=num1+boo1;
console.log(boo);
console.log(typeof boo);
4.number+undefined NAN是一个number NaN:not a number 不是一个数字
var und=num1+undef;
console.log(und);
console.log(typeof und);
5.string+boolean 拼接
var stb=str1+boo1;
console.log(stb);
console.log(typeof stb);
6.string+undefined
var sfind=str1+undef;
console.log(sfind);
console.log(typeof sfind);
