For this example I will use this javascipt :

function demoMatchClick() {
  var re = new RegExp(document.demoMatch.regex.value);
  if (document.demoMatch.subject.value.match(re)) {
    alert("Successful match");
  } else {
    alert("No match");
  }
}

function demoShowMatchClick() {
  var re = new RegExp(document.demoMatch.regex.value);
  var m = re.exec(document.demoMatch.subject.value);
  if (m == null) {
    alert("No match");
  } else {
    var s = "Match at position " + m.index + ":n";
    for (i = 0; i < m.length; i++) {
      s = s + m[i] + "n";
    }
    alert(s);
  }
}

function demoReplaceClick() {
  var re = new RegExp(document.demoMatch.regex.value, "g");
  document.demoMatch.result.value =
    document.demoMatch.subject.value.replace(re,
      document.demoMatch.replacement.value);
}

After packing it from here, I get this

eval(function(p,a,c,k,e,r){e=function(c){return c.toString(a)};if(!''.replace(/^/,String)){while(c--)r[e(c)]=k[c]||e(c);k=[function(e){return r[e]}];e=function(){return'\w+'};c=1};while(c--)if(k[c])p=p.replace(new RegExp('\b'+e(c)+'\b','g'),k[c]);return p}('7 h(){4 a=8 9(1.2.b.3);d(1.2.c.3.5(a)){6("j 5")}e{6("f 5")}}7 k(){4 a=8 9(1.2.b.3);4 m=a.l(1.2.c.3);d(m==o){6("f 5")}e{4 s="p q r "+m.t+":\n";u(i=0;i<m.v;i++){s=s+m[i]+"\n"}6(s)}}7 w(){4 a=8 9(1.2.b.3,"g");1.2.x.3=1.2.c.3.ya,1.2.z.3)}',36,36,'|document|demoMatch|value|var|match|alert|function|new|RegExp||regex|subject|if|else|No||demoMatchClick||Successful|demoShowMatchClick|exec|||null|Match|at|position||index|for|length|demoReplaceClick|result|replace|replacement'.split('|'),0,{}))

To unpack it in Firefox,

• Goto Tools->Error Console (or Ctrl+Shift+J).
• Paste the packed script in input box
• Add “eval = alert;” (without quotes) before the packed script.
• Hit Evaluate, a pop up shows the unpacked script as

• Press Ctrl + A to select all the text. Paste it to notepad.

You got the original javascript back! This is pretty useful while analyzing malicious code to find a solution to counter-attack(or stop) that code.