Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

Cats:  <input name="animal" type="checkbox" value="Cats">
Dogs:  <input name="animal" type="checkbox" value="Dogs">

<p>Click the button to check all checkboxes that have a name attribute with the value "animal".</p>

<button onclick="myFunction()">Try it</button>

<script>
function myFunction() {
    var x = document.getElementsByName("animal");
    var i;
    for (i = 0; i < x.length; i++) {
        if (x[i].type == "checkbox") {
            x[i].checked = true;
        }
    }
}
</script>

</body>
</html>