Added closure solutions

SunJieMing · SunJieMing · commit 1475c94c34ea · 2017-10-02T08:05:18.000-07:00
diff --git a/src/closure.js b/src/closure.js
@@ -1,31 +1,54 @@
 // Complete the following functions.
 
+/* eslint-disable no-unused-vars */
+
 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++;
+    return count;
+  };
 };
 
 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),
+    decrement: () => (--count),
+  };
 };
 
 const limitFunctionCallCount = (cb, n) => {
   // Should return a function that invokes `cb`.
   // The returned function should only allow `cb` to be invoked `n` times.
+  let callCount = 0;
+  return (...args) => {
+    if (callCount === n) return null;
+    callCount++;
+    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 cache = {};
+  return (input) => {
+    if (Object.prototype.hasOwnProperty.call(cache, input)) return cache[input];
+    cache[input] = cb(input);
+    return cache[input];
+  };
 };
 
 /* eslint-enable no-unused-vars */
