Passing A Function Name To Another Funtion

I have a system that I want to try, which requires me, from an aspx
page, to call a generic function to work perform one task, then i want
to call another function with the results of that task.

something like this:

in my aspx page:
<input type=button onClick="doFirstFunction('secondFunctionName');">

then in my .js file:

var functionToCall = ''

function doFirstFunction(functionName) {

functionToCall = functionName

// do the initial, common task
sResult = xmlHttp.responseText

//now call the second function, passing sResult to it

}

Right now I'm using 'switch' and manually adding possible values for
'functionName' then directing manually, but I'd like to make
"doFirstFunction" self-maintaining, i.e. without having to update it if
I add more 'second' functions later.

I thought I could use

eval('functionToCall('+sResult+')');