modify open/closed principle

Author: krzysztof-grzybek

README.md

@@ -1022,38 +1022,84 @@ class UserSettings {
 As stated by Bertrand Meyer, "software entities (classes, modules, functions,
 etc.) should be open for extension, but closed for modification." What does that
 mean though? This principle basically states that you should allow users to
-extend the functionality of your module without having to open up the `.js`
-source code file and manually manipulate it.
+add new functionalities without changing existing code.
 
 **Bad:**
 ```javascript
-class AjaxRequester {
+class AjaxAdapter extends Adapter {
   constructor() {
-    // What if we wanted another HTTP Method, like DELETE? We would have to
-    // open this file up and modify this and put it in manually.
-    this.HTTP_METHODS = ['POST', 'PUT', 'GET'];
+    super();
+    this.name = 'ajaxAdapter';
   }
+}
 
-  get(url) {
-    // ...
+class NodeAdapter extends Adapter {
+  constructor() {
+    super();
+    this.name = 'nodeAdapter';
+  }
+}
+
+class HttpRequester {
+  constructor(adapter) {
+    this.adapter = adapter;
+  }
+
+  fetch(url) {
+    if (this.adapter.name === 'ajaxAdapter') {
+      return makeAjaxCall(url).then(response => {
+        // transform response and return
+      });
+    } else if(this.adapter.name === 'httpNodeAdapter') {
+      return makeHttpCall(url).then(response => {
+        // transform response and return
+      });
+    }
   }
+}
+
+function makeAjaxCall(url) {
+  // request and return promise
+}
 
+function makeHttpCall(url) {
+  // request and return promise
 }
 ```
 
 **Good**:
 ```javascript
-class AjaxRequester {
+class AjaxAdapter extends Adapter {
   constructor() {
-    this.HTTP_METHODS = ['POST', 'PUT', 'GET'];
+    super();
+    this.name = 'ajaxAdapter';
   }
 
-  get(url) {
-    // ...
+  request(url) {
+    // request and return promise
+  }
+}
+
+class NodeAdapter extends Adapter {
+  constructor() {
+    super();
+    this.name = 'nodeAdapter';
   }
 
-  addHTTPMethod(method) {
-    this.HTTP_METHODS.push(method);
+  request(url) {
+    // request and return promise
+  }
+}
+
+class HttpRequester {
+  constructor(adapter) {
+    this.adapter = adapter;
+  }
+
+  fetch(url) {
+    return this.adapter.request(url).then(response => {
+      // transform response and return
+    });
   }
 }
 ```