You can specify an empty string as an argument to join, if no argument is specified a comma is used.

 arr.join('');

http://jsfiddle.net/mowglisanu/CVr25/1/

String.prototype.replace

If you want a simple Array#map, you may use String#replace instead.

s.split`,`.map(v=>f(v))+''      // comma split and join
s.replace(/[^,]+/g,v=>f(v))     // comma separated
s.replace(/\w+/g,v=>f(v))       // comma separated

s.split` `.map(v=>f(v)).join` ` // space split and join
s.replace(/\S+/g,v=>f(v))       // space separated

String#replace may be shorter if you can find a short RegExp. /[^,]+/ is a bit longer, but if /\w+/ is acceptable, it could be shorter.

But be notice that the 2nd parameter of replace callback is the position by character instead of position in array.

You may also use some capture groups to parse the string in it.