generate random strings, e.g., for passwords

random_string.js

/*
in one line for bookmarkletts:
javascript:!function(){"use strict";function r(){var r=new Uint8Array(n);window.crypto.getRandomValues(r);var r=Array.apply([],r);return r=r.filter(function(r){return r>32&&127>r}),String.fromCharCode.apply(String,r)}for(var n=50,t=20,a=r();a.length<t;)a+=r();prompt("",a)}();
*/

(function() {
  "use strict";
  var MAXLEN=50; /* tweak this */
  var MINLEN=20;
  function genString() {
    var array = new Uint8Array(MAXLEN);
    window.crypto.getRandomValues(array);
    var array = Array.apply( [], array ); /* turn into non-typed array */
    array = array.filter(function(x) {
      /* strip non-printables: if we transform into desirable range we have a propability bias, so I suppose we better skip this character */
      return x > 32 && x < 127;
    });
    return String.fromCharCode.apply(String, array);
  }
  var tmp = genString();
  while (tmp.length < MINLEN) {
    /* unlikely too loop more than once.. */
    tmp += genString(); } prompt("",tmp);
})();

Pretty nice. I wanted to constrain to characters that didn't include so many symbols but avoided bias by keeping the set to 64 (128 bits % 64 == 0):

A-Za-z0-9@-

Changed the array to Uint16Array to get 128 bits and the filter to return r===45 || r>=47&&r<=57 || r>=64&&r<=90 || r>=97&&r<=122