JS Easy way to create Random String

jsRandString.js

if (Object['defineProperty'] && !String.hasOwnProperty('rand')) Object.defineProperty(String,"rand",{value:function(){function f(a){for(var c=(Math.random()*eval("1e"+~~(50*Math.random()+50))).toString(36).split(""),b=3;b<c.length;b++)b==~~(Math.random()*b)+1&&c[b].match(/[a-z]/)&&(c[b]=c[b].toUpperCase());c=c.join("");c=c.substr(~~(Math.random()*~~(c.length/3)),~~(Math.random()*(c.length-~~(c.length/3*2)+1))+~~(c.length/3*2));if(24>a)return a?c.substr(c,a):c;c=c.substr(c,a);if(c.length==a)return c;for(;c.length<a;)c+=f();return c.substr(0,a)}var d=arguments,a,e;if(!d.length)return f();for(var b=0;b<d.length;b++)"string"==typeof d[b]&&d[b].length&&!a&&(a=d[b]),"number"==typeof d[b]&&d[b]&&!e&&(e=d[b]);if(!a&&!e)return f();if(!a)return f(e);if(!e){a=window.btoa(escape(encodeURIComponent(a))).replace(/[^\w]/g,"");a=a.split("");for(b=a.length-1;0<b;b--){var d=Math.floor(Math.random()*(b+1)),g=a[b];a[b]=a[d];a[d]=g}return a.join("")}a=window.btoa(escape(encodeURIComponent(a))).replace(/[^\w]/g,"");b=f(e-a.length);a=(a+b).split("");for(b=a.length-1;0<b;b--)d=Math.floor(Math.random()*(b+1)),g=a[b],a[b]=a[d],a[d]=g;a=a.join("");return a.length==e?a:a.substr(0,e)}});

// Use as simple as 
var str = String.rand();

// Accepts 2 parameters, order of no matter
// Param STRING If provided, will encode the string and use letters and numbers for encoding for generating random number
//    If length is longer than encoding, then it will be salted with other random characters
// Param INT If provided, determines total length of the return String. Quite accurate.