string.concat(string2, string3[, ..., stringN]);

var str1 = new String( "This is string one" ); 
var str2 = new String( "This is string two" ); 
var str3 = str1.concat(str2.toString());
console.log("str1 + str2 : "+str3)

str1 + str2 : This is string oneThis is string two