randomStr

randomStr.js

function randomStr(len) {
  var str = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
    strLen = str.length,
    arr = [];
  while (len > 0) {
    var index = Math.random() * strLen,
      a = str.slice(index, index + 1);
    if (a) {
      arr.push(a);
      len--;
    }
  }
  return arr.join('');
}
console.log(randomStr(10));
console.log(randomStr(20));
console.log(randomStr(30));
console.log(randomStr(100));