Updates.

code/hp-react.js

@@ -140,7 +140,42 @@ function renderChildren(element, domElement, prevElement = { props: { children:
     });
 }
 
+function defaultAreEqual(oldProps, newProps) {
+    if (typeof oldProps !== 'object' || typeof newProps !== 'object') {
+        return false;
+    }
 
+    const oldKeys = Object.keys(oldProps);
+    const newKeys = Object.keys(newProps);
+
+    if (oldKeys.length !== newKeys.length) {
+        return false;
+    }
+
+    for (let i = 0; i < oldKeys.length; i++) {
+        // Object.is - the comparison to note
+        if (!oldProps.hasOwnProperty(newKeys[i]) ||
+            !Object.is(oldProps[newKeys[i]], newProps[newKeys[i]])) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+function memo(component, areEqual = defaultAreEqual) {
+    let oldProps = {};
+    let lastResult = false;
+    return (props) => {
+        if (lastResult && areEqual(oldProps, props) {
+            return lastResult;
+        } else {
+            lastResult = component(props);
+            oldProps = Object.assign({}, props); // shallow copy
+            return lastResult;
+        }
+    };
+}
 
 var Hello = memo(({ dateTime }) => {
     return ['h1', {}, [`It is: ${dateTime}`]];