Disable submit button until all form inputs have data

Question

I'm trying to disable the submit button until all inputs have some data. Right now the button is disabled, but it stays disabled after all inputs are filled in. What am I doing wrong?

$(document).ready(function (){
    validate();
    $('input').on('keyup', validate);
});

function validate(){
    if ($('input').val().length > 0) {
        $("input[type=submit]").prop("disabled", false);
    } else {
        $("input[type=submit]").prop("disabled", true);
    }
} 

Answer 1

Here's a modification of your code that checks all the <input> fields, instead of just the first one.

$(document).ready(function() {
  validate();
  $('input').on('keyup', validate);
});

function validate() {
  var inputsWithValues = 0;
  
  // get all input fields except for type='submit'
  var myInputs = $("input:not([type='submit'])");

  myInputs.each(function(e) {
    // if it has a value, increment the counter
    if ($(this).val()) {
      inputsWithValues += 1;
    }
  });

  if (inputsWithValues == myInputs.length) {
    $("input[type=submit]").prop("disabled", false);
  } else {
    $("input[type=submit]").prop("disabled", true);
  }
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="text"><br>
<input type="text"><br>
<input type="text"><br>
<input type="submit" value="Join">

Answer 2

Vanilla JS Solution.

In question selected JavaScript tag.

HTML Form:

<form action="/signup">
    <div>
        <label for="username">User Name</label>
        <input type="text" name="username" required/>
    </div>
    <div>
        <label for="password">Password</label>
        <input type="password" name="password" />
    </div>
    <div>
        <label for="r_password">Retype Password</label>
        <input type="password" name="r_password" />
    </div>
    <div>
        <label for="email">Email</label>
        <input type="text" name="email" />
    </div>
    <input type="submit" value="Signup" disabled="disabled" />
</form>

JavaScript:

var form = document.querySelector('form')
var inputs = document.querySelectorAll('input')
var required_inputs = document.querySelectorAll('input[required]')
var register = document.querySelector('input[type="submit"]')
form.addEventListener('keyup', function(e) {
    var disabled = false
    inputs.forEach(function(input, index) {
        if (input.value === '' || !input.value.replace(/\s/g, '').length) {
            disabled = true
        }
    })
    if (disabled) {
        register.setAttribute('disabled', 'disabled')
    } else {
        register.removeAttribute('disabled')
    }
})

Some explanation:

In this code we add keyup event on html form and on every keypress check all input fields. If at least one input field we have are empty or contains only space characters then we assign the true value to disabled variable and disable submit button.

If you need to disable submit button until all required input fields are filled in - replace:

inputs.forEach(function(input, index) {

with:

required_inputs.forEach(function(input, index) {

where required_inputs is already declared array containing only required input fields.

JSFiddle Demo: https://jsfiddle.net/ydo7L3m7/