How to submit a form with select without Submit button?

Question

Good evening, how do I submit a form with select without the Ubmit button? For example, to create a filter that when selecting the option it automatically submits the form without having the Submit button.

My form is like this:

<form class="form-inline left" method="POST">
            <div class="form-group">
                <label for="listar">Listar por</label>
                <select id="filtro" name="filtro" class="form-control">
                    <option value="professor">Professor</option>
                    <option value="assunto">Assunto</option> 
                </select>
            </div>
        </form>

Answer 1

You can do this directly in select with onchange="this.form.submit()":

<select id="filtro" name="filtro" class="form-control" onchange="this.form.submit()">

That way when the event change is created it sends the form directly.

Example: https://jsfiddle.net/kgfcetud/

Another way is to use an event headphone like this:

document.getElementById('filtro').addEventListener('change', function() {
    this.form.submit();
});

Example: https://jsfiddle.net/rdhcp58d/