JS . Concatenate and substring in same JS statement.

Hi,

I'm my conversion file I have a substring formula:

js: %external%.substring(2,6)

Now I also want to concatenate an "A" infront.

So for example - the value :

XX1234

Should look like this after my conversion:

A1234.

How can I achive this in my conversion file ?

Thank You,

Jdalby