Используя ajax примерно так:
//act="/configs.json"
j=function(a){for(a=0;a<4;a++)try{return a?new ActiveXObject([,"Msxml2","Msxml3","Microsoft"][a]+".XMLHTTP"):new XMLHttpRequest}catch(b){}};
function getJsonData(act)
{
var x=j();
x.open("GET", act);
x.onreadystatechange=function() {...