for in
var color='red';for(var p in color){console.log(color[p])}; //r //e //d var a=null;for(var k in a){console.log(a[k])}; //undefined var flag=true;for(var j in flag){cosole.log(flag[j])}; //undefined var num=12345;for(var m in num){ cosole.log(num[m])}; //undefined
for in 的原理是先判断color的类型,如果color的类型是null或者undefined,则直接跳过该for 循环
若color的类型是原始值,则将其转为它的包装对象。再一一枚举它的属性。每次循环之前都会计算表达式(in前面)的值,并将属性名赋值给它。 (var p=字符串^其实就是属性名;)
var o={x:1,y:2,z:3};
var a=[];
var i=0;
for(a[i++] in o);console.log(a); //["x", "y", "z"]
把o的属性赋值给数组