CIS 119 Compressed nested selection

The code

The HTML:

<p><button id="cow">Cow</button></p>
<p><button id="pig">Pig</button></p>
<p><button id="duck">Duck</button></p>
<p><button id="fox">Fox</button></p>

The JavaScript:

function showSound() {
    var animal = this.id;
    if (animal === 'cow') alert('Mooo');
    else if (animal === 'pig') alert('Oink');
    else if (animal === 'duck') alert('Quack');
    else alert("I don't know. What does the " + animal + " say?");
}

function init() {
    document.getElementById('cow').onclick = showSound;
    document.getElementById('pig').onclick = showSound;
    document.getElementById('duck').onclick = showSound;
    document.getElementById('fox').onclick = showSound;
}

window.onload = init;