Encrypt and decrypt a string in javascript

var chars = {'a':'b','b':'c','c':'a'};
var enc;
var str = "back at it again with the bois!";
window.onload = function() {
enc = str.replace(/[abc]/g, m => chars[m]);
alert(enc);
};

String.prototype.toEncodedString = function(){var ostr=this.toString().replace(/\s+/g,'');if(ostr.length<8){alert("Password must be at least 8 characters long with no spaces.");return null;};var x,nstr='',len=ostr.length;for(x=0;x<len;++x){nstr+=(255-ostr.charCodeAt(x)).toString(36).toUpperCase().toPaddedString(2,'0');};return nstr;};
String.prototype.fromEncodedString = function(){var ostr=this.toString();var x,nstr='',len=ostr.length;for(x=0;x<len;x+=2){nstr+=String.fromCharCode(255-parseInt(ostr.substr(x,2),36));};return nstr;};
Number.prototype.toPaddedString = function(len,pad){len=(len)?Number(len):2;if(isNaN(len)){alert("Padded String 'length' argument is not numeric.");return null;};var dflt=(isNaN(this.toString()))?" ":"0";pad=(pad)?pad.toString().substr(0,1):dflt;var str=this.toString();if(dflt=="0"){while(str.length<len)str=pad+str;};else{while(str.length<len)str+=pad;};return str;};
String.prototype.toPaddedString = Number.prototype.toPaddedString;
//
var str = window.prompt('Enter string to encode:','');
if (str = str.toEncodedString())
{
    str = window.prompt('Enter encoded string:',str);
    str = str.fromEncodedString();
    alert(str);
}
else
{
    alert('Encoding cancelled.');
}