javascript 此 instanceof 错误消息是什么意思？

What does this instanceof error message mean?

提问by pimvdb

I was playing around with instanceofin Chrome but I got an error message. I thinkI know why (you have to supply a function after the instanceofkeyword that is the constructor the object was created with), but the error message seems to be stating something else:

我instanceof在 Chrome 中玩，但我收到一条错误消息。我想我知道为什么（您必须instanceof在创建对象的构造函数的关键字之后提供一个函数），但错误消息似乎说明了其他内容：

[1,2,3] instanceof Array
// true

[1,2,3] instanceof []
// TypeError: Expecting a function in instanceof check, but got 1,2,3

Does this mean that I should replace [1,2,3]with a function? I would think that [1,2,3]is correct and that []is the problem and should be replaced with a function, but it looks like the error message is saying the opposite.

这是否意味着我应该[1,2,3]用函数替换？我认为这[1,2,3]是正确的，这[]就是问题所在，应该用一个函数替换，但看起来错误消息说的正好相反。

Could someone please explain how I'm interpreting the error message incorrectly?

有人可以解释一下我是如何错误地解释错误消息的吗？

回答by user113716

Objects are instances of a constructor function, so the test is to see if the left hand is an instance ofthe right, so the right must be a function (and it must be the constructor that constructed the object to return true).

对象是构造函数的实例，所以测试是看左边是否是右边的实例，所以右边一定是一个函数（并且它必须是构造要返回的对象的构造函数true）。

[1,2,3] instanceof [].constructor;  // true

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So to answer the question more directly, your initial understanding is correct, and the error message seems misleading (to me anyway).

所以为了更直接地回答这个问题，你最初的理解是正确的，错误信息似乎具有误导性（无论如何对我来说）。

From the spec: http://ecma262-5.com/ELS5_HTML.htm#Section_11.8.6

从规范：http: //ecma262-5.com/ELS5_HTML.htm#Section_11.8.6

1.8.6 The instanceof operator

The production RelationalExpression: RelationalExpression instanceof ShiftExpression is evaluated as follows:

• Let lref be the result of evaluating RelationalExpression.
• Let lval be GetValue(lref).
• Let rref be the result of evaluating ShiftExpression.
• Let rval be GetValue(rref).
• If Type(rval) is not Object, throw a TypeError exception.
• If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
• Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

1.8.6 instanceof 运算符

产生式 RelationalExpression: RelationalExpression instanceof ShiftExpression 的计算方式如下：

• 让 lref 是评估 RelationalExpression 的结果。
• 令 lval 为 GetValue(lref)。
• 让 rref 是评估 ShiftExpression 的结果。
• 令 rval 为 GetValue(rref)。
• 如果 Type(rval) 不是 Object，则抛出 TypeError 异常。
• 如果 rval 没有 [[HasInstance]] 内部方法，则抛出 TypeError 异常。
• 返回调用带有参数 lval 的 rval 的 [[HasInstance]] 内部方法的结果。

and http://ecma262-5.com/ELS5_HTML.htm#Section_15.3.5

和http://ecma262-5.com/ELS5_HTML.htm#Section_15.3.5

15.3.5 Properties of Function Instances

15.3.5 函数实例的属性

In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases use a different version of the [[Get]] internal property. Depending on how they are created (see 8.6.2 ,13.2, 15, and 15.3.4.5), function instances may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a [[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal property.

除了所需的内部属性之外，每个函数实例都有一个 [[Call]] 内部属性，并且在大多数情况下使用不同版本的 [[Get]] 内部属性。根据它们的创建方式（见 8.6.2、13.2、15 和 15.3.4.5），函数实例可能有一个 [[HasInstance]] 内部属性、一个 [[Scope]] 内部属性、一个 [[Construct]]内部属性、[[FormalParameters]] 内部属性、[[Code]] 内部属性、[[TargetFunction]] 内部属性、[[BoundThis]] 内部属性和 [[BoundArgs]] 内部属性。

So it requires a TypeErrorif the right hand does not have an internal [[HasInstance]]property, but doesn't specify the wording.

所以它需要一个TypeErrorif 右手没有内部[[HasInstance]]属性，但没有指定措辞。

Firefox 4 gives me a much more sensible error message:

Firefox 4 给了我一个更合理的错误信息：

[1,2,3] instanceof [];

// TypeError: invalid 'instanceof' operand []