Question

134

What does this instanceof error message mean?

rated 0 times [ 141] [ 7] / answers: 1 / hits: 17592 / 13 Years ago, mon, may 16, 2011, 12:00:00

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

[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.

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

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

aman

Add To Favorites

Follow

Total Points: 341

Total Questions: 92

Total Answers: 92

Location: Papua New Guinea

Member since Thu, Jul 9, 2020

4 Years ago

answered 13 Years ago lucianod · Accepted Answer

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

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

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

From the spec: 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.

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

15.3.5 Properties of Function Instances

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.

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

Firefox 4 gives me a much more sensible error message:

[1,2,3] instanceof [];



// TypeError: invalid 'instanceof' operand []