$('#user_input, #pass_input, #v_pass_input, #email').bind('keyup', function() {
if(allFilled()) $('#register').removeAttr('disabled');
});


function allFilled() {
var filled = true;
$('body input').each(function() {
if($(this).val() == '') filled = false;
});
return filled;
}

JSFiddle with your code, works :)

DEMO: http://jsfiddle.net/kF2uK/2/

function buttonState(){
$("input").each(function(){
$('#register').attr('disabled', 'disabled');
if($(this).val() == "" ) return false;
$('#register').attr('disabled', '');
})
}


$(function(){
$('#register').attr('disabled', 'disabled');
$('input').change(buttonState);
})

Check out this jsfiddle.

HTML

// note the change... I set the disabled property right away
<input type="submit" id="register" value="Register" disabled="disabled" />

JavaScript

(function() {
$('form > input').keyup(function() {


var empty = false;
$('form > input').each(function() {
if ($(this).val() == '') {
empty = true;
}
});


if (empty) {
$('#register').attr('disabled', 'disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
} else {
$('#register').removeAttr('disabled'); // updated according to http://stackoverflow.com/questions/7637790/how-to-remove-disabled-attribute-with-jquery-ie
}
});
})()

The nice thing about this is that it doesn't matter how many input fields you have in your form, it will always keep the button disabled if there is at least 1 that is empty. It also checks emptiness on the .keyup() which I think makes it more convenient for usability.

All variables are cached so the loop and keyup event doesn't have to create a jQuery object everytime it runs.

var $input = $('input:text'),
$register = $('#register');
$register.attr('disabled', true);


$input.keyup(function() {
var trigger = false;
$input.each(function() {
if (!$(this).val()) {
trigger = true;
}
});
trigger ? $register.attr('disabled', true) : $register.removeAttr('disabled');
});

Check working example at http://jsfiddle.net/DKNhx/3/

For all solutions instead of ".keyup" ".change" should be used or else the submit button wont be disabled when someone just selects data stored in cookies for any of the text fields.

Grave digging... I like a different approach:

elem = $('form')
elem.on('keyup','input', checkStatus)
elem.on('change', 'select', checkStatus)


checkStatus = (e) =>
elems = $('form').find('input:enabled').not('input[type=hidden]').map(-> $(this).val())
filled = $.grep(elems, (n) -> n)
bool = elems.size() != $(filled).size()
$('input:submit').attr('disabled', bool)

I refactored the chosen answer here and improved on it. The chosen answer only works assuming you have one form per page. I solved this for multiple forms on same page (in my case I have 2 modals on same page) and my solution only checks for values on required fields. My solution gracefully degrades if JavaScript is disabled and includes a slick CSS button fade transition.

See working JS fiddle example: https://jsfiddle.net/bno08c44/4/

JS

$(function(){
function submitState(el) {


var $form = $(el),
$requiredInputs = $form.find('input:required'),
$submit = $form.find('input[type="submit"]');


$submit.attr('disabled', 'disabled');


$requiredInputs.keyup(function () {


$form.data('empty', 'false');


$requiredInputs.each(function() {
if ($(this).val() === '') {
$form.data('empty', 'true');
}
});


if ($form.data('empty') === 'true') {
$submit.attr('disabled', 'disabled').attr('title', 'fill in all required fields');
} else {
$submit.removeAttr('disabled').attr('title', 'click to submit');
}
});
}


// apply to each form element individually
submitState('#sign_up_user');
submitState('#login_user');
});

CSS

input[type="submit"] {
background: #5cb85c;
color: #fff;
transition: background 600ms;
cursor: pointer;
}


input[type="submit"]:disabled {
background: #555;
cursor: not-allowed;
}

HTML

<h4>Sign Up</h4>
<form id="sign_up_user" data-empty="" action="#" method="post">
<input type="email" name="email" placeholder="Email" required>
<input type="password" name="password" placeholder="Password" required>
<input type="password" name="password_confirmation" placeholder="Password Confirmation" required>
<input type="hidden" name="secret" value="secret">
<input type="submit" value="signup">
</form>


<h4>Login</h4>
<form id="login_user" data-empty="" action="#" method="post">
<input type="email" name="email" placeholder="Email" required>
<input type="password" name="password" placeholder="Password" required>
<input type="checkbox" name="remember" value="1"> remember me
<input type="submit" value="signup">
</form>

Built upon rsplak's answer. It uses jQuery's newer .on() instead of the deprecated .bind(). In addition to input, it will also work for select and other html elements. It will also disable the submit button if one of the fields becomes blank again.

var fields = "#user_input, #pass_input, #v_pass_input, #email";


$(fields).on('change', function() {
if (allFilled()) {
$('#register').removeAttr('disabled');
} else {
$('#register').attr('disabled', 'disabled');
}
});


function allFilled() {
var filled = true;
$(fields).each(function() {
if ($(this).val() == '') {
filled = false;
}
});
return filled;
}

Demo: JSFiddle

This works well since all the inputs have to meet the condition of not null.

$(function () {
$('#submits').attr('disabled', true);
$('#input_5').change(function () {
if ($('#input_1').val() != '' && $('#input_2').val() != '' && $('#input_3').val() != '' && $('#input_4').val() != '' && $('#input_5').val() != '') {
$('#submit').attr('disabled', false);
} else {
$('#submit').attr('disabled', true);
}
});
});

If you happen to be using vanilla JavaScript...

let fields = document.querySelectorAll('form > input')
let submitButton = document.querySelector('form > button')


fields = Array.from(fields) // Turn fields into an Array to access the ".every" method.


fields.forEach(field => {
field.addEventListener('keyup', () => {
submitButton.disabled = !fields.every(field => field.value)
})
})

<form>
<input type="text">
<input type="text">
<button type="submit" disabled>Submit</button>
</form>