为什么这个函数在JS中是returnundefined.property？

Question

我正在阅读 MDN JavaScript 指南并尝试与 the example near the bottom of the page 一起工作。

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);

我已将原始代码更改为 return 控制台语句，但是当我将其放入 JSBin.com 时，控制台 returns:

"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？谢谢

Answer 1

您的函数 dump_props(obj, obj_name) 需要第二个 obj_name 参数。但是你称它只是传递了一个参数：

dump_props(car);

因此，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 arguments 太多了。

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中是returnundefined.property？

Why does this function return undefined.property in JS?

javascript

object

undefined