Focus next input once reaching maxlength value

Question

How can I focus the next input once the previous input has reached its maxlength value?

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

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

jsFiddle: http://jsfiddle.net/4m5fg/1/

I must stress that I do not want to use a plugin, as I'd much rather learn the logic behind this, than use something that already exists. Thanks for understanding.

Answer 1

If you're focused on creating card(debit/credit) number input type. Then clean an easily manageable jQuery version as follows:

/*..............................................................................................
* jQuery function for Credit card number input group
......................................................................................................*/
            // make container label of input groups, responsible
            $('.card-box').on('focus', function(e){
                $(this).parent().addClass('focus-form-control');
            });
            $('.card-box').on('blur', function(e){
                $(this).parent().removeClass('focus-form-control');
            });
            $('.card-box-1').on('keyup', function(e){
                e.preventDefault();
                var max_length = parseInt($(this).attr('maxLength'));
                var _length = parseInt($(this).val().length);
                if(_length >= max_length) {
                    $('.card-box-2').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
                if(_length <= 0){
                    return;
                }
            });
            $('.card-box-2').on('keyup', function(e){
                e.preventDefault();
                var max_length = parseInt($(this).attr('maxLength'));
                var _length = parseInt($(this).val().length);
                if(_length >= max_length) {
                    $('.card-box-3').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
                if(_length <= 0){
                    $('.card-box-1').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
            });
             $('.card-box-3').on('keyup', function(e){
                e.preventDefault();
                var max_length = parseInt($(this).attr('maxLength'));
                var _length = parseInt($(this).val().length);
                if(_length >= max_length) {
                    $('.card-box-4').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
                if(_length <= 0){
                    $('.card-box-2').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
            });
            $('.card-box-4').on('keyup', function(e){
                e.preventDefault();
                var max_length = parseInt($(this).attr('maxLength'));
                var _length = parseInt($(this).val().length);
                if(_length >= max_length) {
                    return;
                }
                if(_length <= 0){
                    $('.card-box-3').focus().removeAttr('readonly');
                    $(this).attr('readonly', 'readonly');
                }
            });
/*..............................................................................................
* End jQuery function for Credit card number input group
......................................................................................................*/

/* Hide HTML5 Up and Down arrows. */
                                input[type="number"]::-webkit-outer-spin-button, input[type="number"]::-webkit-inner-spin-button {
                                    -webkit-appearance: none; margin: 0;
                                }
                                input[type="number"] { -moz-appearance: textfield; }
                                .card-box {
                                    width: 20%; display: inline-block; height: 100%; border: none;
                                }
                                
                                .focus-form-control {
                                    border-color: #66afe9; outline: 0;-webkit-box-shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px rgba(102, 175, 233, .6);
                                        box-shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px rgba(102, 175, 233, .6);
                                }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
<div class="form-control" style="padding: 0; max-width: 300px; ">
                                        <input class="card-box card-box-1" type="number" id="CreditCard_CardNumber1" required step="1" minlength="4" maxlength="4" pattern="[0-9]{4}" value="" placeholder="0000"
                                            onClick="this.setSelectionRange(0, this.value.length)" oninput="this.value=this.value.slice(0,this.maxLength||'');this.value=(this.value < 1) ? ('') : this.value;"/> 
                                        <input class="card-box card-box-2" type="number" id="CreditCard_CardNumber2" readonly required step="1" minlength="4" maxlength="4" pattern="[0-9]{4}" value="" placeholder="0000"
                                            onClick="this.setSelectionRange(0, this.value.length)" oninput="this.value=this.value.slice(0,this.maxLength||'');this.value=(this.value < 1) ? ('') : this.value;" />
                                        <input class="card-box card-box-3" type="number" id="CreditCard_CardNumber3" readonly required step="1" minlength="4" maxlength="4" pattern="[0-9]{4}" value="" placeholder="0000"
                                            onClick="this.setSelectionRange(0, this.value.length)" oninput="this.value=this.value.slice(0,this.maxLength||'');this.value=(this.value < 1) ? ('') : this.value;" />
                                        <input class="card-box card-box-4" type="number" id="CreditCard_CardNumber4" readonly required step="1" minlength="4" maxlength="4" pattern="[0-9]{4}" value="" placeholder="0000"
                                            onClick="this.setSelectionRange(0, this.value.length)" oninput="this.value=this.value.slice(0,this.maxLength||'');this.value=(this.value < 1) ? ('') : this.value;" />
                                    </div>

Answer 2

You can use plain JavaScript:

See DEMO.

Check the character length with el.value.length. If it is equal to the maximum value, move to the next field by using focus(). Bind this function to the keyup event with onkeyup so that the function fires every time after the user keys in a character.

var a = document.getElementById("a"),
    b = document.getElementById("b"),
    c = document.getElementById("c");

a.onkeyup = function() {
    if (this.value.length === parseInt(this.attributes["maxlength"].value)) {
        b.focus();
    }
}

b.onkeyup = function() {
    if (this.value.length === parseInt(this.attributes["maxlength"].value)) {
        c.focus();
    }
}