How could an undefined variable throw a type error?

Question

I have a user who is getting the error

TypeError: a is undefined

I'm confused how this can happen. Won't trying to access an undefined variable throw a reference error? In what situation could it throw a type error?

Answer 1

As pointed out by @jgillich in his answer, the following code produces a TypeError on an undefined object.

> a
ReferenceError: a is not defined
> var a;
> a.x
TypeError: a is undefined

To understand why, we can refer to ECMAScript 5.1 spec section 11.2.1 Property Accessors. We are interested in step 5

5. Call CheckObjectCoercible(baseValue).

In our example, baseValue is the value of the reference a. That means baseValue is undefined.

CheckObjectCoercible is defined in section 9.10

The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 15:

And we can see in Table 15 that a TypeError is thrown for undefined and null values.

So the reason why we have a TypeErrorinstead of a ReferenceError is, as usual, because the spec says so!

There are other ways to obtain a TypeError on undefined, notably ToObject also throws a TypeError for undefined.

These three lines of code produce TypeError: can't convert undefined to object:

Object.defineProperties({}, undefined);
Object.prototype.toLocaleString.call(undefined);
Object.prototype.valueOf.call(undefined);

Though this time the message is a bit clearer.

Also invoking directly on undefined produces TypeError: undefined has no properties

undefined.foo();
undefined.x;

All of this was tested using Firefox 33.0a2 (Aurora).

Answer 2

> a
ReferenceError: a is not defined
> var a;
> a.x
TypeError: a is undefined