머라카노?

var ar  = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

var result;

for(var i = 0; i < 6; i++) {

  result += ar.charAt(Math.floor(Math.random() * ar.length));

};

console.log( result );