From: FAQ server on
FAQ Topic - How do I access a property of an object using
a string?

There are two ways to access properties: the dot notation and
the square bracket notation. What you are looking for is the square
bracket notation in which the dot, and the identifier to its right,
are replaced with a set of square brackets containing a string. The
value of the string matches the identifier. For example:-

//dot notation
var bodyElement = document.body;

//square bracket notation, using an expression
var bodyElement = document["bo"+"dy"];

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From: Dr J R Stockton on
In comp.lang.javascript message <4b6a0e09$0$275$14726298(a)
>, Thu, 4 Feb 2010 00:00:03, FAQ server <javascript(a)>
>//square bracket notation, using an expression
>var bodyElement = document["bo"+"dy"];

That should be replaced by something that looks useful, similar to

var formNumberJ = document.forms["Frm" + J]

(c) John Stockton, nr London UK. ?? Turnpike v6.05 MIME.
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