Referencing Array-syntax-like Form Variable

I am experience some problems reading a form variable from a
Javascript function. The point with this particular variable is that
its name has the following syntax:

<input name="tx_impexp[tt_content:159]"/>

I want to set this var to 1 from my javascript function, so I tried
to execute the following line of code:

document.frm_1.tx_impexp[exclude][tt_content:159].value=1;

Unfortunately the following error come up:

Error: missing ] in index expression
document.frm_1.tx_impexp[exclude][tt_content:159].value=1
--------------------------------------------------------------|

It seems Javascript expects to close down the bracket at the position
marked...An easy solution would be to change my var syntax but the
point is that I cannot since I am using an already made script and I
should not modify it.

Just wandering if this is due to a syntax error defined according to
Javascript specification language or there is something wrong with
this.