Your jQuery selector isn't going to find that form, since the selector is running before the form tag exists in the DOM. Try wrapping it in the jQuery function to wait for the document to be ready:
$(function () {
$('#form_id').on('submit', function(e){
// the rest of your code
});
});
It also might be a good idea to return false at the end, to further suppress the form's default post action:
e.preventDefault();
$.ajax({
type: "POST",
url: "./postinfo.php",
data: $(this).serialize(),
success: function() {
alert('success');
}
});
return false;
Currently the form is posting as normal. Since the AJAX handler is never attached (because the element doesn't exist when the selector executes), it's just doing a normal document-level form post. And since there's no action attribute specified in the form tag, the page is posting to itself by default. Which just responds with the current page.
Edit: You also have a typo which may be preventing the browser from executing your JavaScript code at all:
<script type="text/javscript">
You're missing the second "a". This should be:
<script type="text/javascript">
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