My ajax code not returning ajax value

In above case your ajax request is not reaching your php function, thus it founds nothing. Update your jquery function with this

 function jqueryfunction(id) {
    var value = jQuery(this).val();
    if (value.length > 3) {
        jQuery.ajax({
            url: ajaxurl,
            type: 'POST',
            data: {
                action: 'mycustomfunction',
                value: value,
                },
            success: function (data) {
                console.log(data);
            }
        });
    } else {
        $('.showcount').remove();
    }
}

as you can see that i added a parameter “action” which is name of your ajax request after wp_ajax_.

Also update your php function with

function mycustomfunction() {
    echo "Function found";
die; //wp_die();
}

Add die at end of your function , otherwise you will see a “0” with your ajax response.

Hope it will help you.