Javascript "Cannot read property 'length' of undefined" when checking a variable's length

You can check that theHref is defined by checking against undefined.

if (undefined !== theHref && theHref.length) {
    // `theHref` is not undefined and has truthy property _length_
    // do stuff
} else {
    // do other stuff
}

If you want to also protect yourself against falsey values like null then check theHref is truthy, which is a little shorter

if (theHref && theHref.length) {
    // `theHref` is truthy and has truthy property _length_
}

Why?

You asked why it happens, let's see:

The official language specificaion dictates a call to the internal [[GetValue]] method. Your .attr returns undefined and you're trying to access its length.

If Type(V) is not Reference, return V.

This is true, since undefined is not a reference (alongside null, number, string and boolean)

Let base be the result of calling GetBase(V).

This gets the undefined part of myVar.length .

If IsUnresolvableReference(V), throw a ReferenceError exception.

This is not true, since it is resolvable and it resolves to undefined.

If IsPropertyReference(V), then

This happens since it's a property reference with the . syntax.

Now it tries to convert undefined to a function which results in a TypeError.

There's a difference between an empty string "" and an undefined variable. You should be checking whether or not theHref contains a defined string, rather than its lenght:

if(theHref){
   // ---
}

If you still want to check for the length, then do this:

if(theHref && theHref.length){
   // ...
}