diff --git a/html/jquery.couponcode.js b/html/jquery.couponcode.js
index 1347dd6..101cc18 100644
--- a/html/jquery.couponcode.js
+++ b/html/jquery.couponcode.js
@@ -1,7 +1,9 @@
+/*jshint forin:true, noarg:true, noempty:true, eqeqeq:true, bitwise:true, strict:true, undef:true, unused:true, curly:true, browser:true, jquery:true, indent:4, laxbreak:true, maxerr:500 */
+/*global alert:true */
 /*
  * couponCode - jQuery plugin
  *
- * Copyright (c) 2011 Grant McLean (grant@mclean.net.nz)
+ * Copyright (c) 2011-2017 Grant McLean (grant@mclean.net.nz)
  *
  * Dual licensed under the MIT and GPL licenses:
  *   http://www.opensource.org/licenses/mit-license.php
@@ -9,155 +11,158 @@
  *
  */
 
-(function($) {
+(function ($) {
+    'use strict';
 
-var SYMBOL_SET = '0123456789ABCDEFGHJKLMNPQRTUVWXY';
-var BAD_SYMBOL = new RegExp('[^' + SYMBOL_SET + ']');
+    var SYMBOL_SET = '0123456789ABCDEFGHJKLMNPQRTUVWXY';
+    var BAD_SYMBOL = new RegExp('[^' + SYMBOL_SET + ']');
 
-$.fn.couponCode = function(options) {
-    return this.each(function() {
-        $.fn.couponCode.build(this, options);
-    });
-};
-
-$.fn.couponCode.build = function(base_entry, options) {
-    var self    = $.extend({}, $.fn.couponCode.defaults, options);
-    self.focus  = null;
-    self.inputs = [];
-    self.flags  = [];
-    self.parts  = parseInt(self.parts, 10);
-    if(isNaN(self.parts) || self.parts < 1 || self.parts > 6) {
-        alert("CouponCode 'parts' must be in range 1-6");
-        return;
-    }
+    $.fn.couponCode = function (options) {
+        return this.each(function () {
+            $.fn.couponCode.build(this, options);
+        });
+    };
 
-    var start_val = $(base_entry).val();
-    var name    = base_entry.name;
-    var id      = base_entry.id;
-    var wrapper = $( $(base_entry).wrap('<span class="jq-couponcode" />').parent()[0] );
-    wrapper[0].removeChild(base_entry);
-    var hidden  = $('<input type="hidden">').attr({ 'name': name, 'id': id });
-    wrapper.append(hidden);
-    var inner   = $('<span class="jq-couponcode-inner" />');
-    for(var i = 0; i < self.parts; i++) {
-        if(i > 0) {
-            inner.append($('<span class="jq-couponcode-sep" />').text(self.separator));
+    $.fn.couponCode.build = function (base_entry, options) {
+        var self    = $.extend({}, $.fn.couponCode.defaults, options);
+        self.focus  = null;
+        self.inputs = [];
+        self.flags  = [];
+        self.parts  = parseInt(self.parts, 10);
+        if (isNaN(self.parts) || self.parts < 1 || self.parts > 6) {
+            alert("CouponCode 'parts' must be in range 1-6");
+            return;
         }
-        self.inputs[i] = $('<input type="text" class="jq-couponcode-part" />');
-        inner.append(self.inputs[i]);
-    }
-    $( self.inputs ).each(function(i, input) {
-        input
-        .keydown(function() { setTimeout(function() { validate(i); }, 5 ); } )
-        .blur(function() { self.focus = null; validate(i); } )
-        .focus( function() { self.focus = i; } );
-    });
-    self.inputs[0].on('paste', function() {
-        setTimeout(function() { set_parts(self.inputs[0].val()); }, 2);
-    });
 
-    if(start_val.length > 0) {
-        set_parts(start_val);
-    }
-    wrapper.append(inner);
-    if(self.setFocus) {
-        self.inputs[0].focus();
-    }
+        var start_val = $(base_entry).val();
+        var name      = base_entry.name;
+        var id        = base_entry.id;
+        var wrapper   = $($(base_entry).wrap('<span class="jq-couponcode" />').parent()[0]);
+        wrapper[0].removeChild(base_entry);
+        var hidden    = $('<input type="hidden">').attr({ 'name': name, 'id': id });
+        wrapper.append(hidden);
+        var inner     = $('<span class="jq-couponcode-inner" />');
+        for (var i = 0; i < self.parts; i++) {
+            if (i > 0) {
+                inner.append($('<span class="jq-couponcode-sep" />').text(self.separator));
+            }
+            self.inputs[i] = $('<input type="text" class="jq-couponcode-part" />');
+            inner.append(self.inputs[i]);
+        }
+        $(self.inputs).each(function (i, input) {
+            input
+                .keydown(function () { setTimeout(function () { validate(i); }, 5); })
+                .blur(function ()    { self.focus = null; validate(i); })
+                .focus(function ()   { self.focus = i; });
+        });
+        self.inputs[0].on('paste', function () {
+            setTimeout(function () { set_parts(self.inputs[0].val()); }, 2);
+        });
 
-    function set_parts(code) {
-        var parts = code.split('-');
-        for(var i = 0; i < self.parts; i++) {
-            self.inputs[i].val( parts[i] || '');
-            validate(i);
+        if (start_val.length > 0) {
+            set_parts(start_val);
+        }
+        wrapper.append(inner);
+        if (self.setFocus) {
+            self.inputs[0].focus();
         }
-    }
 
-    function validate(index) {
-        var input  = self.inputs[index];
-        var result = validate_one_field(input, index);
-        if(result === true) {
-            input.removeClass('jq-couponcode-bad');
-            input.addClass('jq-couponcode-good');
-            self.flags[index] = 1;
+        function set_parts(code) {
+            var parts = code.split('-');
+            for (var i = 0; i < self.parts; i++) {
+                self.inputs[i].val(parts[i] || '');
+                validate(i);
+            }
         }
-        else {
-            input.removeClass('jq-couponcode-good');
-            input.removeClass('jq-couponcode-bad');
-            if(result === false) {
+
+        function validate(index) {
+            var input  = self.inputs[index];
+            var result = validate_one_field(input, index);
+            if (result === true) {
+                input.removeClass('jq-couponcode-bad');
+                input.addClass('jq-couponcode-good');
+                self.flags[index] = 1;
+            }
+            else if (result === false) {
+                input.removeClass('jq-couponcode-good');
                 input.addClass('jq-couponcode-bad');
+                self.flags[index] = 0;
             }
-            self.flags[index] = 0;
+            else {// result === null
+                input.removeClass('jq-couponcode-good');
+                input.removeClass('jq-couponcode-bad');
+                self.flags[index] = 0;
+            }
+            update_hidden_field();
         }
-        update_hidden_field();
-    }
 
-    function validate_one_field(input, index) {
-        var val = input.val();
-        var focussed = (self.focus === index);
-        if(val == '') { return; }
-        var code = clean_up( val );
-        if(code.length > 4 || BAD_SYMBOL.test(code)) {
-            return false;
-        }
-        if(code.length < 4) {
-            return focussed ? null : false;
-        }
-        if(code.charAt(3) != checkdigit(code, index + 1)) {
-            return false;
-        }
-        if(val != code) {
-            input.val(code);
-        }
-        if(focussed && (index < self.parts - 1)) {
-            self.inputs[index + 1].focus();
-            self.focus = index + 1;
+        function validate_one_field(input, index) {
+            var val      = input.val();
+            var focussed = (self.focus === index);
+            if (val === '') { return; }
+            var code = clean_up(val);
+            if (code.length > 4 || BAD_SYMBOL.test(code)) {
+                return false;
+            }
+            if (code.length < 4) {
+                return focussed ? null : false;
+            }
+            if (code.charAt(3) !== checkdigit(code, index + 1)) {
+                return false;
+            }
+            if (val !== code) {
+                input.val(code);
+            }
+            if (focussed && (index < self.parts - 1)) {
+                self.inputs[index + 1].focus();
+                self.focus = index + 1;
+            }
+            return true;
         }
-        return true;
-    }
 
-    function clean_up(code) {
-        code = code.toUpperCase()
-               .replace(/ /g, '')
-               .replace(/O/g, '0')
-               .replace(/I/g, '1')
-               .replace(/S/g, '5')
-               .replace(/Z/g, '2');
-        return code;
-    }
-
-    function checkdigit(data, pos) {
-        var check = pos;
-        for(var i = 0; i < 3; i++) {
-            var k = SYMBOL_SET.indexOf( data.charAt(i) );
-            check = check * 19 + k;
+        function clean_up(code) {
+            code = code.toUpperCase()
+                .replace(/ /g, '')
+                .replace(/O/g, '0')
+                .replace(/I/g, '1')
+                .replace(/S/g, '5')
+                .replace(/Z/g, '2');
+            return code;
         }
-        return SYMBOL_SET.charAt(check % 31);
-    }
 
-    function update_hidden_field() {
-        var parts    = [];
-        var old_code = hidden.val();
-        $( self.inputs ).each(function(i, input) {
-            parts.push(input.val());
-        });
-        var new_code = parts.join('-');
-        if(old_code == new_code) {
-            return;
+        function checkdigit(data, pos) {
+            var check = pos;
+            for (var i = 0; i < 3; i++) {
+                var k = SYMBOL_SET.indexOf(data.charAt(i));
+                check = check * 19 + k;
+            }
+            return SYMBOL_SET.charAt(check % 31);
         }
-        hidden.val(new_code);
-        if(self.onChange) {
-            var good_parts = 0;
-            $( self.flags ).each(function(i, val) { good_parts = good_parts + val; });
-            self.onChange(good_parts === self.parts);
+
+        function update_hidden_field() {
+            var parts    = [];
+            var old_code = hidden.val();
+            $(self.inputs).each(function (i, input) {
+                parts.push(input.val());
+            });
+            var new_code = parts.join('-');
+            if (old_code === new_code) {
+                return;
+            }
+            hidden.val(new_code);
+            if (self.onChange) {
+                var good_parts = 0;
+                $(self.flags).each(function (i, val) { good_parts = good_parts + val; });
+                self.onChange(good_parts === self.parts);
+            }
         }
-    }
 
-};
+    };
 
-$.fn.couponCode.defaults = {
-    parts     : 3,
-    separator : '-',
-    setFocus  : false
-};
+    $.fn.couponCode.defaults = {
+        parts     : 3,
+        separator : '-',
+        setFocus  : false
+    };
 
 })(jQuery);