lab15: init

lab15/smart-array.js

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+"use strict";
+
+// Matches paterns like '3-10'
+const RANGE_PAT = /^(\d+)-(\d+)$/;
+
+// Matches negative index values
+const FROM_END_PAT = /^-(\d+)$/;
+
+const NUM_PAT = /^-?\d+$/;
+
+function SmartArray(...args) {
+  return new Proxy(args, {
+    get: function(target, prop) {
+      if (prop.match(RANGE_PAT)) {
+        // Return a subarray of the elements in the specified range,
+        // INCLUDING the specified end index.
+        let start = parseInt(prop.replace(RANGE_PAT, "$1"));
+        let end = parseInt(prop.replace(RANGE_PAT, "$2")) + 1;
+        return target.slice(start, end);
+      } else if (prop.match(FROM_END_PAT)) {
+        //
+        // ***YOUR CODE HERE***
+        //
+        // Return the element at the specified position, counting
+        // back from target.length.  So "-1" will refer to the last
+        // element in the array.
+        //
+        // If the resulting index position is negative,
+        // raise an exception.
+      } else {
+        // Do the usual array thing -- get the value at the specified index.
+        return Reflect.get(...arguments);
+      }
+    },
+    set: function(target, prop, newVal) {
+      //
+      // ***YOUR CODE HERE***
+      //
+      // For smart arrays, we only allow updates to numerical fields.
+      // Throw an exception for a 'prop' that is not a valid integer.
+      //
+      // If prop is zero or positive, update the array position normally.
+      //
+      // If negative, update the position counting from the end of the array.
+      // However, raise an exception if the resulting index is still negative.
+    },
+  });
+}
+
+let arr = SmartArray('a', 'b', 'c', 'd', 'e', 'f');
+
+console.log(arr[0]); // a
+console.log(arr[4]); // e
+console.log(arr['hello']); // undefined
+
+console.log(arr['2-4']); // [c,d,E]
+console.log(arr['3-5']); // [d,E,f]
+
+console.log(arr[-1]); // f
+console.log(arr[-3]); // d
+
+try {
+  console.log(arr[-99]);
+} catch (e) {
+  console.log("Exception correctly thrown.");
+}
+
+arr[1] = 'B';
+console.log(arr[1]); // B
+
+arr[-2] = 'E';
+console.log(arr[4]); // E
+
+try {
+  arr['2-4'] = 'hello';
+} catch (e) {
+  console.log("Exception correctly thrown.");
+}
+
+try {
+  arr[3*"hello"] = 'hello';
+} catch (e) {
+  console.log("Exception correctly thrown.");
+}
+
+console.log(arr);

lab15/tracing.js

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+let obj = { foo: 'bar' };
+
+let o = new Proxy(obj, {
+  set: (target, name, val) => {
+    console.log(`Setting ${name} to ${val}`);
+    //target[name] = val;
+    //return true;
+    //return Reflect.set(target,name,val);
+    return Reflect.set(...arguments);
+  },
+  get: (target, name) => {
+    console.log(`Getting ${name}`);
+    return Reflect.get(...arguments);
+  }
+});
+
+o.foo = "fighters";
+
+let ff = "foo" + o.foo;
+
+

lab15/undoable.js

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+"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}`);
+