Read javascript variable from plugin

Hi, in a html5 plugin I'm trying to check if a javascript variable is true or false :

in my xml a button to set the variable to true :

onclick="js(set_onlytext)"

in my html the javascript function:

function set_onlytext() {onlytext= true;}

and in my js plugin two different action if variable is true or false:

if (!onlytext) { Do something }
else if (onlytext) { Do something else}

looks like from the plugin the variable is always set to false...
how to do this ?

Thanx !