/**

 * RANDOM STRING GENERATOR

 *

 * Info:      http://stackoverflow.com/a/27872144/383904

 * Use:       randomString(length [,"A"] [,"N"] );

 * Default:   return a random alpha-numeric string

 * Arguments: If you use the optional "A", "N" flags:

 *            "A" (Alpha flag)   return random a-Z string

 *            "N" (Numeric flag) return random 0-9 string

 */

function randomString(len, an){

    an = an&&an.toLowerCase();

    var str="", i=0, min=an=="a"?10:0, max=an=="n"?10:62;

    for(;i++<len;){

      var r = Math.random()*(max-min)+min <<0;

      str += String.fromCharCode(r+=r>9?r<36?55:61:48);

    }

    return str;

}

​

​

​

​

​

​

​

for(var i=0; i<20; i++){

  console.log( randomString(4, "N") );

}

​

​

​