Solution 1:

var mystr = "Doe";
mystr = "John " + mystr;

Wouldn't this work for you?

Solution 2:

You could do it this way ..

var mystr = 'is my name.';
mystr = mystr.replace (/^/,'John ');

console.log(mystr);

disclaimer: http://xkcd.com/208/


Wait, forgot to escape a space.  Wheeeeee[taptaptap]eeeeee.