我试图禁用提交按钮，做检查和给予警告消息，我不能弄清楚我做错了，任何帮助将不胜感激。

<form id="post_code">
        <input name="textfield" type="text" id="postcode" size="14" maxlength="8" />
        <input type="image" id="submit_postcode" src="images/B_go.png" alt="Submit Postcode" width="59" height="24" border="0" />
  </form>


$(function() {

   $('form#post_code').submit(function() {

    var entered_post_code = $('form#post_code input#postcode').val();
    var post_code = "CV";
    if (entered_post_code == post_code) {
        alert("post code is ok");
        return true;
    }else {
        alert("post code is not ok");
        return true;
    }
    return false;   
});

});