diff --git a/index.js b/index.js
index e69de29..547f035 100644
--- a/index.js
+++ b/index.js
@@ -0,0 +1,47 @@
+// Demonstrate polymorphism.
+function HtmlElement() {
+    this.click = function() {
+        console.log('click');
+    }
+}
+
+HtmlElement.prototype.focus = function() {
+    console.log('focus');
+}
+
+// Select Element.
+function HtmlSelectElement(items = []) {
+    this.items = items;
+
+    this.addItem = function(item) {
+        this.items.push(item);
+    }
+
+    this.removeItem = function(item) {
+        this.items.pop(item);
+    }
+
+    this.render = function() {
+        let selectHtml = '<select>';
+        for (let value of this.items) {
+            selectHtml += `<option>${value}</option>`;
+        }
+        selectHtml += '</select>';
+        return selectHtml;
+    }
+}
+
+HtmlSelectElement.prototype = new HtmlElement();
+HtmlSelectElement.prototype.constructor = HtmlSelectElement;
+
+// Image Element.
+function HtmlImageElement(src = 'https://image') {
+    this.src = src;
+
+    this.render = function () {
+        return `<img src="${this.src}" />`;
+    }
+}
+
+HtmlImageElement.prototype = new HtmlElement();
+HtmlImageElement.prototype.constructor = HtmlImageElement;