Hide/locking a <form> button until form is completely filled with validation

Question

I make my JSON script with a form and a button, but when I demo it on someone, they can press the submit button whenever they like. Here is my code:

<form action="action_page.php" method="GET">
  <label for="fname">First name:</label>
  <input type="text" id="fname" name="fname"<br><br>
  <label for="lname">Last name:</label>
  <input type="text" id="lname" name="lname"<br><br>
</form>

All I need now is validation and how to lock or hide the button.

I tried using <script> with function and getElementById but I simply do not know how to lock it. Here is the message that will be inputted when it is locked and the form isn't completed:

You have not finished the information input for the survey. Please input "Your First Name" and "Your Last Name" to enter the survey.

When they finish, I will input a loading icon until fully loaded.

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Code Language(s)

I use fiddles to make my code, so I use:

• HTML
• JSON
• Pinch of jQuery (loading icon)
  

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Answer Expectations

In my answers, I need:

• Recommended Code Language
• Code for note purposes
• Crossed out attributes
• Attributes before coding explanations

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EDIT: I just noticed that I can use the disabled boolean attribute here, so all I need is validation to disable the boolean attribute so they can press the button.

Answer 1

Hello I recommend JavaScript but it is not secure at all. However its the most effective in this case. Here is the code: (Note that i changed the html too)

<form action="action_page.php" method="get" >
  <label for="fname">First name:</label>
  <input type="text" id="fname" name="fname" onkeyup="inputEntries()"><br><br>
  <label for="lname">Last name:</label>
  <input type="text" id="lname" name="lname" onkeyup="inputEntries()"><br><br>
  <button type="submit" name="submit" id="submit_btn" disabled="true">submit</button>
</form>

<script>
  var fname_var = document.getElementById("fname");
  var lname_var = document.getElementById("lname");

  var submit_btn_var = document.getElementById("submit_btn");

  function inputEntries(){
    if(lname_var.value != "" && fname_var.value != "")
        submit_btn_var.disabled = false;
    else
      submit_btn_var.disabled = true;
  }
</script>

What it does is that if both input fields are empty the button is not clickable but if both are it can be clicked. hope it helped!