ajax post回调函数不起作用

Here is index.php

这是index . php

post_data = {'username':username, 'old_pass':old_pass, 'new_pass':new_pass};

$.post('change_pass.php', post_data, function(result) {
    if(result.text == 'success') {
        $("#message").text("Password changed successfully");
    } else {
        $("#message").text("Error changing password");
    }
}, 'json');
post_data =