Convert String Into object using javascript

Convert String Into object using javascript:

How to get a input value from the string using javascript.
Use "eval" function to get input value from the string.

Example:

<form>
  <input type="text" name="test" id="test" value="1" />
  <input type="button" name="te" onclick="testing()" />
</form>

<script>
  function testing() {
   var st = "document.getElementById('test').value";

   alert(st);

//output:  document.getElementById('test').value
  //output : 1

   alert(eval(st));

  //output : 1
  }
</script>