Focus next input once reaching maxlength value

No jQuery used and is a very clean implementation:

• Reads from the maxlength attribute.
• Scales to any number of inputs inside of your container.
• Automatically finds the next input to focus.
• No jQuery.

http://jsfiddle.net/4m5fg/5/

<div class="container">
a: <input type="text" maxlength="5" />
b: <input type="text" maxlength="5" />
c: <input type="text" maxlength="5" />
</div>

..

var container = document.getElementsByClassName("container")[0];
container.onkeyup = function(e) {
    var target = e.srcElement || e.target;
    var maxLength = parseInt(target.attributes["maxlength"].value, 10);
    var myLength = target.value.length;
    if (myLength >= maxLength) {
        var next = target;
        while (next = next.nextElementSibling) {
            if (next == null)
                break;
            if (next.tagName.toLowerCase() === "input") {
                next.focus();
                break;
            }
        }
    }
    // Move to previous field if empty (user pressed backspace)
    else if (myLength === 0) {
        var previous = target;
        while (previous = previous.previousElementSibling) {
            if (previous == null)
                break;
            if (previous.tagName.toLowerCase() === "input") {
                previous.focus();
                break;
            }
        }
    }
}

You can watch for input in the fields and test its value:

$("input").bind("input", function() {
    var $this = $(this);
    setTimeout(function() {
        if ( $this.val().length >= parseInt($this.attr("maxlength"),10) )
            $this.next("input").focus();
    },0);
});

Working demo.

The setTimeout is there to ensure the code will only run after the input is completed and the value updated. Binding input ensures most types of input will trigger the event, including key presses, copy/paste (even from mouse) and drag & drop (though in this case, the latter won't work, since the focus was on the draggable, not the droppable).

Note: on some older browsers, you might also need to bind propertychange.

---

If a user pastes text that is greater than the maxlength, ideally it should spill into the next input.

To do that, you might need to remove the maxlength attribute using JavaScript (to be able to capture the full input), and implement that functionality yourself. I made a small example, relevant parts below:

$("input").each(function() {
    var $this = $(this);
    $(this).data("maxlength", $this.prop("maxlength"));
    $(this).removeAttr("maxlength");
})

This removes the attribute, but saves it in data, so you can access it later.

function spill($this, val) {
    var maxlength = $this.data("maxlength");
    if ( val.length >= maxlength ) {
        $this.val(val.substring(0, maxlength));
        var next = $this.next("input").focus();
        spill(next, val.substring(maxlength));
    }
    else
        $this.val(val);
}

Here the max length logic is reintroduced in JavaScript, as well as getting the "discarded" part and using it in a recursive call to spill. If there's no next element, the call to data will return undefined and the loop will stop, so the input will be truncated in the last field.