.4

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);