Response to http://stackoverflow.com/q/35840215/1166087

HTML

<input type="text" class="required" /><br>
<input type="text" class="required" /><br>
<input type="submit" id="submit_button">

JavaScript

$(document).ready(function() {
    var button = $('#submit_button');
    var required = $('.required');
    var counter = 0;
    var trigger = required.length;
    button.hide();

    required.on('keyup', function () {
        var self = $(this);
        if(self.val() !== '') {
            if (!self.data('valid') && ++counter === trigger) button.show();
            self.data('valid', true);
        } else {
            if (self.data('valid')) --counter;
            button.hide();
            self.data('valid', false);
        }
    });
});