lab15: init

lab15/undoable.js

@@ -0,0 +1,47 @@
+"use strict";
+
+// Input: any object.
+//
+// Returns: An object with a 'revert' method that reverts
+//    any changes to the specified field.
+function makeUndoable(o) {
+  let oldVals = {};
+  return new Proxy(o, {
+    get: (target, name) => {
+      // Adding a 'revert' method on undoable objects,
+      // which is not part of the original object.
+      if (name === 'revert') {
+        return function(p) {
+          target[p] = oldVals[p];
+        }
+        // If any property other than 'revert' is requested,
+        // return the value from the underlying object.
+      } else return target[name];
+    },
+    set: (target, name, val) => {
+      oldVals[name] = target[name];
+      o[name] = val;
+      // Returns true to indicate that assignment was successful.
+      return true;
+    },
+    deleteProperty: (target, name) => {
+      oldVals[name] = target[name];
+      return delete target[name];
+    },
+  });
+}
+
+let emp = { fname: "Joe", lname: "Smith", id: 1234, salary: 100 };
+emp = makeUndoable(emp);
+
+emp.fname = "Joey";
+console.log(`Joe's name accidentally changed to ${emp.fname}`);
+
+emp.revert('fname');
+console.log(`Joe's name changed back to ${emp.fname}`);
+
+delete emp.lname;
+console.log(`${emp.fname}'s last name is now ${emp.lname}`);
+emp.revert('lname');
+console.log(`${emp.fname}'s last name is now ${emp.lname}`);
+