How does an undefined variable throw a type error?

As @jgillich pointed out in his answer, the following code produces one TypeErroron the undefinedobject .

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

To see why, see Section 11.2.1, "Property Accessors" of the ECMAScript 5.1 specification . We are interested in step 5

5. Call CheckObjectCoercible( baseValue ).

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

CheckObjectCoercibleDefined in Section 9.10

Raises an error if the argument to the abstract operation CheckObjectCoercible is a value that cannot be converted to Object using ToObject. Defined by Table 15:

We can see in Table 15 that one TypeErroris thrown undefinedand the nullvalue.

So , as usual, the reason we use a TypeErrorinstead of a ReferenceErroris because the spec says so!

There are other ways to get TypeErroron undefined, in particular ToObject can also throw TypeErrorfor 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);

Although this time the message is clearer.

Also called directly on the undefinedproductTypeError: undefined has no properties

undefined.foo();
undefined.x;

All tested with Firefox 33.0a2 (Aurora).