extra polyfill required for IE11

polyfill.js

@@ -142,6 +142,61 @@ if (!Object.is) {
     };
 }
 
+/*! https://mths.be/codepointat v0.2.0 by @mathias */
+if (!String.prototype.codePointAt) {
+    (function() {
+        'use strict'; // needed to support `apply`/`call` with `undefined`/`null`
+        var defineProperty = (function() {
+            // IE 8 only supports `Object.defineProperty` on DOM elements
+            try {
+                var object = {};
+                var $defineProperty = Object.defineProperty;
+                var result = $defineProperty(object, object, object) && $defineProperty;
+            } catch(error) {}
+            return result;
+        }());
+        var codePointAt = function(position) {
+            if (this == null) {
+                throw TypeError();
+            }
+            var string = String(this);
+            var size = string.length;
+            // `ToInteger`
+            var index = position ? Number(position) : 0;
+            if (index != index) { // better `isNaN`
+                index = 0;
+            }
+            // Account for out-of-bounds indices:
+            if (index < 0 || index >= size) {
+                return undefined;
+            }
+            // Get the first code unit
+            var first = string.charCodeAt(index);
+            var second;
+            if ( // check if it’s the start of a surrogate pair
+                first >= 0xD800 && first <= 0xDBFF && // high surrogate
+                size > index + 1 // there is a next code unit
+            ) {
+                second = string.charCodeAt(index + 1);
+                if (second >= 0xDC00 && second <= 0xDFFF) { // low surrogate
+                    // https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
+                    return (first - 0xD800) * 0x400 + second - 0xDC00 + 0x10000;
+                }
+            }
+            return first;
+        };
+        if (defineProperty) {
+            defineProperty(String.prototype, 'codePointAt', {
+                'value': codePointAt,
+                'configurable': true,
+                'writable': true
+            });
+        } else {
+            String.prototype.codePointAt = codePointAt;
+        }
+    }());
+}
+
 Math.log10 = Math.log10 || function(x) {
     return Math.log(x) * Math.LOG10E;
 };