For-in loop in JavaScript is used to iterate over the properties of an object. It can be a great debugging tool if we want to show the contents of an object. The for-in loop iterates only over those keys of an object which have their enumerable property set to “true”. The key values in an object have four attributes (value, writable, enumerable, and configurable). Enumerable when set to “true” means that we can iterate over that property. 


Syntax 

 for (key in object) {
       // code block to be executed
}
<!DOCTYPE html>
<html>
<head>
  <title>JavaScript</title>
  <script>
    var obj = {
      firstName : "Yahoo",
      lastName : "Baba",
      Age : 25,
      email : "hello@yahoobaba.net"
    };

    for(var key in obj){
      document.write(key + " : " + obj[key] + "<br>");
    }
  </script>
</head>
<body>
</body>
</html>