@@ -1,38 +1,62 @@
-// Complete the following functions.
-
-const counter = () => {
-  // Return a function that when invoked increments and returns a counter variable.
-  // Example: const newCounter = counter();
-  // newCounter(); // 1
-  // newCounter(); // 2
-};
-
-const counterFactory = () => {
-  // Return an object that has two methods called `increment` and `decrement`.
-  // `increment` should increment a counter variable in closure scope and return it.
-  // `decrement` should decrement the counter variable and return it.
-};
-
-const limitFunctionCallCount = (cb, n) => {
-  // Should return a function that invokes `cb`.
-  // The returned function should only allow `cb` to be invoked `n` times.
-};
-
-/* Extra Credit */
-const cacheFunction = (cb) => {
-  // Should return a funciton that invokes `cb`.
-  // A cache (object) should be kept in closure scope.
-  // The cache should keep track of all arguments have been used to invoke this function.
-  // If the returned function is invoked with arguments that it has already seen
-  // then it should return the cached result and not invoke `cb` again.
-  // `cb` should only ever be invoked once for a given set of arguments.
-};
-
-/* eslint-enable no-unused-vars */
-
-module.exports = {
-  counter,
-  counterFactory,
-  cacheFunction,
-  limitFunctionCallCount
-};
+// Complete the following functions.
+
+const counter = () => {
+  // Return a function that when invoked increments and returns a counter variable.
+  // Example: const newCounter = counter();
+  // newCounter(); // 1
+  // newCounter(); // 2
+  let count = 0;
+  return () => count += 1;
+};
+
+const counterFactory = () => {
+  // Return an object that has two methods called `increment` and `decrement`.
+  // `increment` should increment a counter variable in closure scope and return it.
+  // `decrement` should decrement the counter variable and return it.
+  let count = 0;
+  return {
+    increment: () => count += 1,
+    decrement: () => count -= 1
+  };
+};
+
+const limitFunctionCallCount = (cb, n) => {
+  // Should return a function that invokes `cb`.
+  // The returned function should only allow `cb` to be invoked `n` times.
+  let count = 0;
+  return (...args) => {
+    if (count === n) {
+      return null;
+    }
+    count += 1;
+    return cb(...args);
+  };
+};
+
+/* Extra Credit */
+const cacheFunction = (cb) => {
+  // Should return a funciton that invokes `cb`.
+  // A cache (object) should be kept in closure scope.
+  // The cache should keep track of all arguments have been used to invoke this function.
+  // If the returned function is invoked with arguments that it has already seen
+  // then it should return the cached result and not invoke `cb` again.
+  // `cb` should only ever be invoked once for a given set of arguments.
+  const storage = {};
+  
+    return (input) => {
+      if (Object.prototype.hasOwnProperty.call(storage, input)) {
+        return storage[input];
+      }
+      storage[input] = cb(input);
+      return storage[input];
+    };
+};
+
+/* eslint-enable no-unused-vars */
+
+module.exports = {
+  counter,
+  counterFactory,
+  cacheFunction,
+  limitFunctionCallCount
+};