JSFiddle - React, Tailwind, and code Playground

HTML

<div class="form-group">
    <label for="txtusername" class="col-sm-4 control-label">Username</label>
    <div class="col-sm-8">
        <input type="text" class="form-control" id="txtusername" name="txtusername" placeholder="Username" value="a" />
    </div>
</div>
<div class="form-group">
    <label for="txtpassword" class="col-sm-4 control-label">Password</label>
    <div class="col-sm-8">
        <input  type="password" class="form-control" id="txtpassword" name="txtpassword" placeholder="Password" value="a" />
    </div>
</div>
<div class="form-group">
    <div class="col-sm-offset-4 col-sm-8">
        <div class="checkbox">
            <label><input type="checkbox" />Remember me</label>
        </div>
    </div>
</div>
<div class="form-group">
    <div class="col-sm-offset-4 col-sm-8">
        <button type="submit" id="signin" class="btn btn-default">Sign in</button>
    </div>
</div>

JavaScript

var inputs = [].slice.call(document.getElementsByTagName('input')),
    register = document.getElementById('signin');
function check() {
    register.disabled = !inputs.every(function(inp) {
        return inp.value;
    });
}
$(inputs).on('input', check);
check();