var JS_ver  = [];
(Number.prototype.toFixed)?JS_ver.push("1.5"):false;
([].indexOf && [].forEach)?JS_ver.push("1.6"):false;
((function(){try {[a,b] = [0,1];return true;}catch(ex) {return false;}})())?JS_ver.push("1.7"):false;
([].reduce && [].reduceRight && JSON)?JS_ver.push("1.8"):false;
("".trimLeft)?JS_ver.push("1.8.1"):false;
JS_ver.supports = function()
{
if (arguments[0])
 
return (!!~this.join().indexOf(arguments[0] +",") +",");
else
 
return (this[this.length-1]);
}
alert("Latest Javascript version supported: "+ JS_ver.supports());
alert("Support for version 1.7 : "+ JS_ver.supports("1.7"));

<script type="text/javascript">
var n = navigator;
var u = n.userAgent;
 
var apn = n.appName;
var v = n.appVersion;
var ie = v.indexOf('MSIE ')
if(ie > 0){
   apv = parseInt(i = v.substring(ie + 5));
   if(apv > 3) {
       apv = parseFloat(i);
   }
}else{
   apv = parseFloat(v);
}
 
var isie = (apn == 'Microsoft Internet Explorer');
var ismac = (u.indexOf('Mac') >= 0);
 
var javascriptVersion = "1.0";
 
if(String && String.prototype){
   javascriptVersion = '1.1';
 
   if(javascriptVersion.match){
       javascriptVersion = '1.2';
 
       var tm = new Date;
       if(tm.setUTCDate){
           javascriptVersion = '1.3';
 
           if(isie && ismac && apv >= 5) javascriptVersion = '1.4';
 
           var pn = 0;
           if(pn.toPrecision){
               javascriptVersion = '1.5';
 
               a = new Array;
               if(a.forEach){
                   javascriptVersion = '1.6';
 
                   i = 0;
                   o = new Object;
                   tcf = new Function('o','var e,i=0;try{i=new Iterator(o)}catch(e){}return i');
                   i = tcf(o);
                   if(i && i.next) {
                       javascriptVersion = '1.7';
                   }
               }
           }
       }
   }
}
alert(javascriptVersion);
</script>