JavaScript form validation - to check for an empty textbox - using Python and CGI

Question

I am developing a web application using Python and CGI. For form validation, I intend to use JavaScript. Here is the scenario - There is a textbox where in user enters the name of the project and clicks the submit button. I want to catch if nothing is entered in the textbox and throw an alert.

For the JavaScript function, I have used parameters, project_form (for the form) and project_name for the textbox. But what I am not clear about is where to put this function. As of now, I am trying to put it in the form tag itself. But it is not working. I somehow think that I have to pass the project_form and project_name parameters to the function and due to th e code structure, it is not possible for me to put the function in the 'head' section. I am pasting my code here. Thanks for the help.

<form action="create_project.py" method="POST" name="project_form">
  <input type="text" name="project_name" size="40" onclick="javascript:document.project_form.submit.disabled=false"
  id="acpro_inp0">
  <br>
  <script type="text/javascript">
    function ValidateTextbox(project_form, project_name) {
      var tb_value = project_form.project_name.value;
      if (tb_value == null || tb_value.trim() == "") {
        alert("Please enter something");
        project_form.project_name.focus();
        return false;
      }
      return true;
    }
  </script>
  return ValidateTextbox('project_form','project_name')
  <p>
  </p>
  <input type="submit" name="submit" value="Submit" onsubmit="return ValidateTextbox('project_form','project_name')"
  disabled="">
</form>

Here is the related Python code -

import yate
import jshelper

print yate.start_response()
print yate.include_header("Create a new project.")
#Section to create a new project
print yate.start_form("create_project.py","project_form")
print yate.text_box("project_name",form_name="project_form")
jshelper.JSTextBoxValidate("project_form","project_name")
print yate.end_form("Submit",is_disabled = 'True', onsubmit_callback = "return ValidateTextbox('project_form','project_name')")
print yate.include_footer(({"Home": "../index.html"}))

Here is another piece of Python code, where I try to create the JS function on the fly.

def JSTextBoxValidate(form_name,tb_name):
    print '<script type="text/javascript">'
    print 'function ValidateTextbox' + '(' + form_name + ',' + tb_name + ')'
    print '{' 
    print 'var tb_value = ' + form_name + '.' + tb_name + '.' + 'value;' 
    print 'if (tb_value==null || tb_value.trim()=="")'
    print '{' 
    print 'alert("Please enter something");'
    print form_name + '.' + tb_name + '.' + 'focus();' 
    print 'return false;'
    print '}' 
    print 'return true;'
    print '}'
    print '</script>'

Somehow I have a feeling that I am probably not on the right track in this process and there has to be a better way. So I will be thankful for helping me to get this thing to work. Note - The package yate is from Head First Python and I am using and extending the template from that code. I have put similar note in my project's documentation. This is my personal project.

Answer 1

Rewriting your HTML code the way it should work:

<script type="text/javascript">
  function ValidateTextbox(input) { //we take an object representation of the HTML <input> tag (not just a string)
    if (input.value.trim() == "") { //the attribute value is already defined in the tag now, so it couldn't be null
      alert("Please enter something");
      input.focus(); //move the cursor to the incriminated input
      return false; //preventing from submitting the form (assuming the function is called as a return value)
    }
    return true; //allowing the form to be submitted (assuming the function is called as a return value)
  }
  //function added a you seemed to need something like that
  function DisableFormSubmit(theForm, e) { //we take the form object as argument, and e may be the event object given from the keypress event
    var submitButton=theForm.submit; //to ease typing this code, the submit button is assigned to a local variable
    if(theForm.project_name.value.trim()=='') { //if we have no value for the project_name input of the form
      submitButton.disabled=true; //we disable the submit button
      submitButton.value="Submit - enter a name first"; //and we give a hint why it is disabled in the text printed on the button
    } else { //in this case project_name has a valid value
      submitButton.disabled=false; //the submit button is enabled
      submitButton.value="Submit"; //we remove the hint we gave
    }
    //processing the event now, to know what to return (key 13 is Enter, returning false on it will prevent the form from being submitted by pressing Enter)
    if(window.event) //MSIE compatibility
      return window.event.keyCode!=13;
    else if(e) //web browser's behavior
      return e.which!=13;
    }
  }
</script>
<form action="create_project.py" method="POST" name="project_form" onsubmit="return ValidateTextBox(this.project_name);">
  <input type="text" name="project_name" value="" size="40" onkeypress="return DisableFormSubmit(this.form, event);" id="acpro_inp0">
  <br>

  <input type="submit" name="submit" value="">
</form>
<script type='text/javascript'>
  DisableFormSubmit(document.forms['project_form']); //we call this function once after the <form> has loaded so that we init the submit button with a standard way
</script>

For the Python code, I can't tell as I don't know this language, but you don't need to create as JS function "on the fly" to do a client-side check of the form's validity. Mind the client-side thing, meaning not secure, meaning if you absolutely need a value in that field, do a second check server-side (with Python apparently). Your problem is that:

• you try to make a Javascript instruction out of <script></script> tags
• you give strings as arguments while you actually need object representations of HTML tags: instead of giving 'project_form' and 'project_name' - notice the quotes - you needed something like document.project_form or document.forms['project_form'] and one of the previous with .project_name appended. Notice the lack of quotes here.
• you used the attribute onsubmit in an <input> while its place should be in the <form> (this point may not be a cause of error though)

Hope this helps ;)