adding

demo/demo.css

@@ -1,160 +1 @@
-/*! normalize.css v8.0.1 | MIT License | github.com/necolas/normalize.css */
+/*! normalize.css v8.0.1 | MIT License | github.com/necolas/normalize.css */html{line-height:1.15;-webkit-text-size-adjust:100%}body{margin:0}main{display:block}h1{font-size:2em;margin:.67em 0}hr{box-sizing:content-box;height:0;overflow:visible}pre{font-family:monospace,monospace;font-size:1em}a{background-color:transparent}abbr[title]{border-bottom:none;text-decoration:underline;-webkit-text-decoration:underline dotted;text-decoration:underline dotted}b,strong{font-weight:bolder}code,kbd,samp{font-family:monospace,monospace;font-size:1em}small{font-size:80%}sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}sub{bottom:-.25em}sup{top:-.5em}img{border-style:none}button,input,optgroup,select,textarea{font-family:inherit;font-size:100%;line-height:1.15;margin:0}button,input{overflow:visible}button,select{text-transform:none}[type=button],[type=reset],[type=submit],button{-webkit-appearance:button}[type=button]::-moz-focus-inner,[type=reset]::-moz-focus-inner,[type=submit]::-moz-focus-inner,button::-moz-focus-inner{border-style:none;padding:0}[type=button]:-moz-focusring,[type=reset]:-moz-focusring,[type=submit]:-moz-focusring,button:-moz-focusring{outline:1px dotted ButtonText}fieldset{padding:.35em .75em .625em}legend{box-sizing:border-box;color:inherit;display:table;max-width:100%;padding:0;white-space:normal}progress{vertical-align:baseline}textarea{overflow:auto}[type=checkbox],[type=radio]{box-sizing:border-box;padding:0}[type=number]::-webkit-inner-spin-button,[type=number]::-webkit-outer-spin-button{height:auto}[type=search]{-webkit-appearance:textfield;outline-offset:-2px}[type=search]::-webkit-search-decoration{-webkit-appearance:none}::-webkit-file-upload-button{-webkit-appearance:button;font:inherit}details{display:block}summary{display:list-item}[hidden],template{display:none}
-html {
-  line-height: 1.15;
-  -webkit-text-size-adjust: 100%;
-}
-
-body {
-  margin: 0;
-}
-
-main {
-  display: block;
-}
-
-h1 {
-  font-size: 2em;
-  margin: 0.67em 0;
-}
-
-hr {
-  box-sizing: content-box;
-  height: 0;
-  overflow: visible;
-}
-
-pre {
-  font-family: monospace, monospace;
-  font-size: 1em;
-}
-
-a {
-  background-color: transparent;
-}
-
-abbr[title] {
-  border-bottom: none;
-  text-decoration: underline;
-  -webkit-text-decoration: underline dotted;
-          text-decoration: underline dotted;
-}
-
-b, strong {
-  font-weight: bolder;
-}
-
-code, kbd, samp {
-  font-family: monospace, monospace;
-  font-size: 1em;
-}
-
-small {
-  font-size: 80%;
-}
-
-sub, sup {
-  font-size: 75%;
-  line-height: 0;
-  position: relative;
-  vertical-align: baseline;
-}
-
-sub {
-  bottom: -0.25em;
-}
-
-sup {
-  top: -0.5em;
-}
-
-img {
-  border-style: none;
-}
-
-button, input, optgroup, select, textarea {
-  font-family: inherit;
-  font-size: 100%;
-  line-height: 1.15;
-  margin: 0;
-}
-
-button, input {
-  overflow: visible;
-}
-
-button, select {
-  text-transform: none;
-}
-
-[type=button], [type=reset], [type=submit], button {
-  -webkit-appearance: button;
-}
-
-[type=button]::-moz-focus-inner, [type=reset]::-moz-focus-inner, [type=submit]::-moz-focus-inner, button::-moz-focus-inner {
-  border-style: none;
-  padding: 0;
-}
-
-[type=button]:-moz-focusring, [type=reset]:-moz-focusring, [type=submit]:-moz-focusring, button:-moz-focusring {
-  outline: 1px dotted ButtonText;
-}
-
-fieldset {
-  padding: 0.35em 0.75em 0.625em;
-}
-
-legend {
-  box-sizing: border-box;
-  color: inherit;
-  display: table;
-  max-width: 100%;
-  padding: 0;
-  white-space: normal;
-}
-
-progress {
-  vertical-align: baseline;
-}
-
-textarea {
-  overflow: auto;
-}
-
-[type=checkbox], [type=radio] {
-  box-sizing: border-box;
-  padding: 0;
-}
-
-[type=number]::-webkit-inner-spin-button, [type=number]::-webkit-outer-spin-button {
-  height: auto;
-}
-
-[type=search] {
-  -webkit-appearance: textfield;
-  outline-offset: -2px;
-}
-
-[type=search]::-webkit-search-decoration {
-  -webkit-appearance: none;
-}
-
-::-webkit-file-upload-button {
-  -webkit-appearance: button;
-  font: inherit;
-}
-
-details {
-  display: block;
-}
-
-summary {
-  display: list-item;
-}
-
-template {
-  display: none;
-}
-
-[hidden] {
-  display: none;
-}

src/FormValidator.js

@@ -88,6 +88,12 @@ class FormValidator
      *  @return {[type]}       [description]
      */
     onFieldUpdate(event) {
+
+        // if value is a empty string make him undefined
+        if (!event.detail.value) {
+            event.detail.value = undefined
+        }
+
         let errors = validate.single(event.detail.value, this.constraits[event.detail.name])
         let element = this.findElementByName(event.detail.name)