Bookmarklet to create a eval'ing text area and bootstrapping code to provide some styling/formatting niceties.

gistfile1.js

try {
 
var RIGHTCURLYBRACE="}",LEFTCURLYBRACE="{";
  
  // bookmarklet to kick things off
  //javascript:function%20runCode(e)%20%7Be.preventDefault();eval(document.getElementById(%22codearea%22).value);return%20false;%7D;(function()%7Ba=document.createElement(%22div%22);b=document.body.appendChild(a);b.style.background=%22%23eeeeee%22;b.style.width=%22100%25%22;b.style.height=%22100px%22;b.innerHTML='%3Ctextarea%20id=%22codearea%22%3E%3C/textarea%3E%3Cbutton%20id=%22runcodebutton%22%3ERun%3C/button%3E';document.getElementById('runcodebutton').addEventListener(%22click%22,runCode);a.style.position=%22absolute%22;a.style.top=%220px%22;document.getElementById(%22codearea%22).value=window.localStorage.getItem(%22codeMSG%22);a.style.zIndex=%221000%22%7D)();
  
  function bootstrap() {
   
   // get ourself 
   var codeArea=document.getElementById("codearea");
   
   function trim(text) {
    text=text.replace(/(^\s*)|(\s*$)/gi,"");
   text=text.replace(/[ ]{2,}/gi," ");
    text=text.replace(/\n /,"\n");
    return text;
   }
   
   function indent(text) {
    
    var lines=text.split("\n");
    var currentDepth=0;
    
    for(var i=0;i<lines.length;++i) {
     
     var line=lines[i];
     // have to use constants to avoid the line containing the token itself getting split 
     currentDepth-=line.split(RIGHTCURLYBRACE).length-1;
     
     if(currentDepth>0){
      line=Array(currentDepth+1).join(" ")+line;
      lines[i]=line;
     }
     
     currentDepth+=line.split(LEFTCURLYBRACE).length-1;
    }
    return lines.join("\n");
   }
   
   codeArea.value=trim(codeArea.value);
   
   //thisArea.value=thisArea.value.replace(/{/g,"{\n");
   // thisArea.value=thisArea.value.replace(/}/g,"} \n");
   //thisArea.value=thisArea.value.replace(/;/g,";\n");
   
   var runButton=document.getElementById("runcodebutton");
   runButton.style.width="100%";
   runButton.style.height="80px";
   runButton.style.border="1px solid black";
   runButton.style.backgroundColor="#ffefbb";
   runButton.style.fontSize="40px";
   runButton.style.position="absolute";
   runButton.style.top=0;
   
   // ensure style
   codeArea.style.width="100%";
   codeArea.style.height="550px";
   codeArea.style.fontFamily="courier new";
   codeArea.style.fontSize="20px";
   codeArea.style.position="absolute";
   codeArea.style.top="80px";
   
   // disable auto capitalisation and other 'helpful' things 
   codeArea.autocapitalize="off";
   codeArea.autocorrect="off";
   codeArea.autocomplete="off";
   codeArea.spellcheck="false";
   
   // flash the background blue for 100ms at end of function
   codeArea.style.backgroundColor="grey";
   function resetBackground(){
    codeArea.style.backgroundColor="white";
   }
   
   setTimeout(resetBackground,100);
   
   // auto reindents the code 
   codeArea.value=indent(codeArea.value);
   
   // Save ourselves! 
   window.localStorage.setItem("codeMSG",codeArea.value);
   
   alert(localStorage.items);
  }
  
  bootstrap();
  
} catch(e){
  alert(e);
}