move misc to js/ui

1-js/99-js-misc/01-proxy/02-array-negative/solution.md

@@ -0,0 +1,19 @@
+
+```js run
+let array = [1, 2, 3];
+
+array = new Proxy(array, {
+  get(target, prop, receiver) {
+    if (prop < 0) {
+      // even if we access it like arr[1]
+      // prop is a string, so need to convert it to number
+      prop = +prop + target.length;
+    }
+    return Reflect.get(target, prop, receiver);
+  }
+});
+
+
+alert(array[-1]); // 3
+alert(array[-2]); // 2
+```