add day 3

day3/lib/ReactiveObject.js

@@ -0,0 +1,38 @@
+export class Reactive {
+    constructor(obj) {
+        this.contents = obj;
+        this.listeners = {};
+        this.makeReactive(obj);
+    }
+
+    makeReactive(obj) {
+        Object.keys(obj).forEach(prop => this.makePropReactive(obj, prop));
+    }
+
+    makePropReactive(obj, key) {
+        let value = obj[key];
+
+        // Gotta be careful with this here
+        const that = this;
+
+        Object.defineProperty(obj, key, {
+            get() {
+                return value;
+            },
+            set(newValue) {
+                value = newValue;
+                that.notify(key)
+            }
+        })
+    }
+
+    listen(prop, handler) {
+        if (!this.listeners[prop]) this.listeners[prop] = [];
+
+        this.listeners[prop].push(handler);
+    }
+
+    notify(prop) {
+        this.listeners[prop].forEach(listener => listener(this.contents[prop]));
+    }
+}