Question

我正在浏览MDN JavaScript指南，并尝试与the example near the bottom of the page一起使用。

var car = { make: "Ford", model: "Mustang" };

function dump_props(obj, obj_name) {
   var result = "";
   for (var i in obj) {
      console.log(result += obj_name + "." + i + " = " + obj[i] + "<br>");
   }
   result += "<hr>";
   console.log(result);
}

dump_props(car);

我已经修改了原始代码以返回控制台语句但是当我把它放在JSBin.com中时，控制台返回：

"undefined.make = Ford<br>"
"undefined.make = Ford<br>undefined.model = Mustang<br>"
"undefined.make = Ford<br>undefined.model = Mustang<br><hr>"

为什么未定义？感谢

Answer 1

您的函数dump_props(obj, obj_name)需要第二个obj_name参数。但你称它只是传递一个参数：

dump_props(car);

所以，第二个将是undefined。我想这就是你想要的：

var car = { make: "Ford", model: "Mustang" };

function dump_props(obj, obj_name) {
   var result = "";
   obj_name = obj_name || obj.constructor.name; // Get the object name
   for (var i in obj) {
      console.log(result += obj_name + "." + i + " = " + obj[i] + "<br>");
   }
   result += "<hr>";
   console.log(result);
}

dump_props(car);
dump_props(car,"Car"); // Both ways will work

EXAMPLE

Answer 2

你没有将参数传递给函数尝试

dump_props(car,"carclass");

Answer 3

你的dump_props函数中有一个arguments太多。

var car = { make: "Ford", model: "Mustang" };

function dump_props(obj) {
   var result = "";
   for (var key in obj) {
      console.log(result += key + " = " + obj[key] + "<br>");
   }
   result += "<hr>";
   console.log(result);
}

dump_props(car);

为什么这个函数在JS中返回undefined.property？

3 个答案: