Need to fill a textfield from a form but can't access the field. My script repeats all 6 seconds so as soon as the input field is visible it should be found?

Any idea? Thanks everyone

script:

    $(document).ready(function() {
var a = document.getElementById("field_7");
var b = document.getElementById("postbox-container-1");
if(a)
  console.log("field_7 exists");
else
  console.log("field_7 does not exist"); 
if(b)
  console.log("postbox-container-1 exists");
else
  console.log("postbox-container-1 does not exist");

setTimeout(arguments.callee, 6000);

})();

HTML:

<div class="required zip-code control-group active valid">
<label class="control-label" for="field_7">Postal code / ZIP</label>
<div class="controls" name="controls"><input maxlength="10" type="text" id="field_7"></div></div>