Restructure to allow for adding new chapters

decorator/src/index-01.js

@@ -1,6 +1,6 @@
 function slow(x) {
-  // there can be a heavy CPU-intensive job here
   console.info(`Called with ${x}`);
+  for (let i = 1; i < 1000000000; i++) {}; // CPU-intensive "job"
   return x;
 }
 

decorator/src/index-02.js

@@ -0,0 +1,31 @@
+let worker = {
+  someMethod() {
+    return 1;
+  },
+
+  slow(x) {
+    console.info("Called with " + x);
+    for (let i = 1; i < 1000000000; i++) {}; // CPU-intensive "job"
+    return x * this.someMethod(); // (*)
+  }
+};
+
+function cachingDecorator(func) {
+  let cache = new Map();
+  return function(x) {
+    if (cache.has(x)) {
+      console.info("Cache hit!");
+      return cache.get(x);
+    }
+    console.info("Cache miss!");
+    // let result = func(x); // What's `this`?
+    let result = func.call(this, x); // `this` is passed correctly now
+    cache.set(x, result);
+    return result;
+  };
+}
+
+worker.slow = cachingDecorator(worker.slow); // now make it caching
+
+console.info( worker.slow(2) ); // works
+console.info( "Again: " + worker.slow(2) ); // works, doesn't call the original (cached)

decorator/src/index-03.js

@@ -0,0 +1,34 @@
+let worker = {
+    slow(min, max) {
+        console.info(`Called with ${min},${max}`);
+        for (let i = 0; i < 1000000000; i++) {};
+        return min + max;
+    }
+};
+
+function cachingDecorator(func, hash) {
+    let cache = new Map();
+    return function() {
+        let key = hash(arguments); // (*)
+        if (cache.has(key)) {
+            console.info("Cache hit!");
+            return cache.get(key);
+        }
+
+        let result = func.call(this, ...arguments); // (**)
+
+        cache.set(key, result);
+        console.info("Cache miss!");
+        return result;
+    };
+}
+
+function hash(args) {
+    return args[0] + ',' + args[1];
+}
+
+worker.slow = cachingDecorator(worker.slow, hash);
+
+console.info( worker.slow(3, 5) ); // works
+console.info( worker.slow(8, 7, 9) ); // works, ignoring extra argument
+console.info( "Again " + worker.slow(3, 5) ); // same (cached)