参考错误：引用未定义的属性 "x"

¥Message

ReferenceError: reference to undefined property "x" (Firefox)

¥Error type

（仅限 Firefox）仅当 javascript.options.strict 首选项设置为 true 时才会报告 ReferenceError 警告。

¥(Firefox only) ReferenceError warning which is reported only if javascript.options.strict preference is set to true.

¥What went wrong?

该脚本尝试访问不存在的对象属性。有两种方法可以访问属性；请参阅 属性访问器 参考页以了解有关它们的更多信息。

¥The script attempted to access an object property which doesn't exist. There are two ways to access properties; see the property accessors reference page to learn more about them.

¥Examples

¥Invalid cases

在这种情况下，属性 bar 是未定义的属性，因此会出现 ReferenceError。

¥In this case, the property bar is an undefined property, so a ReferenceError will occur.

const foo = {};
foo.bar; // ReferenceError: reference to undefined property "bar"

¥Valid cases

为了避免该错误，你需要将 bar 的定义添加到对象中，或者在尝试访问它之前检查 bar 属性是否存在；方法包括使用 in 运算符或 Object.hasOwn() 方法，如下所示：

¥To avoid the error, you need to either add a definition for bar to the object or check for the existence of the bar property before trying to access it; ways to do that include using the in operator, or the Object.hasOwn() method, like this:

const foo = {};

// Define the bar property

foo.bar = "moon";
console.log(foo.bar); // "moon"

// Test to be sure bar exists before accessing it

if (Object.hasOwn(foo, "bar")) {
  console.log(foo.bar);
}

¥See also

• 属性访问器