为什么每个JS对象都有一个未定义的属性

Question

let obj = {a:'a',b:'b'};
for (let p in obj){
   console.log(p);
}

输出：

a 
b
undefined

未定义的属性是什么？

Answer 1

strong>打印。

示例：

console.logger = function(p){
           console.log(p);
           return "logged"
      }
var obj = {a:'a',b:'b'};
for (var p in obj){
   console.logger(p);
}

a          // logs the property
b          // logs the property
"logged"   // logs the return of logger function, undefined in case nothing's returned