How can I insert a character after every n characters in javascript?

Keep it simple

  var str = "123456789";
  var parts = str.match(/.{1,3}/g);
  var new_value = parts.join("-"); //returns 123-456-789

With regex

"The quick brown fox jumps over the lazy dogs.".replace(/(.{5})/g,"$1$")

The q$uick $brown$ fox $jumps$ over$ the $lazy $dogs.$

cheers,

function chunk(str, n) {
    var ret = [];
    var i;
    var len;

    for(i = 0, len = str.length; i < len; i += n) {
       ret.push(str.substr(i, n))
    }

    return ret
};

chunk("The quick brown fox jumps over the lazy dogs.", 5).join('$');
// "The q$uick $brown$ fox $jumps$ over$ the $lazy $dogs."