SRP

Author: ryanmcdermott

README.md

@@ -710,6 +710,63 @@ bankAccount.withdraw(100);
 
 
 ## **Classes**
+### Classes should obey Single Responsibility Principle (SRP)
+As stated in Clean Code, "There should never be more than one reason for a class
+to change". It's tempting to jam-pack a class with a lot of functionality, like
+when you can only take one suitcase on your flight. The issue with this is
+that your class won't be conceptually cohesive and it will give it many reasons
+to change. Minimizing the amount of times you need to change a class is important.
+It's important because if too much functioanlity is in one class and you modify a piece of it,
+it can be difficult to understand how that will affect other dependent modules in
+your codebase.
+
+**Bad:**
+```javascript
+class UserSettings {
+  constructor(user) {
+    this.user = user;
+  }
+  
+  changeSettings(settings) {
+    if (this.verifyCredentials(user)) {
+      // ...
+    }
+  }
+
+  verifyCredentials(user) {
+    // ...
+  }
+}
+```
+
+**Good**:
+```javascript
+class UserAuth {
+  constructor(user) {
+    this.user = user;
+  }
+
+  verifyCredentials() {
+    // ...
+  }
+}
+
+
+class UserSettings {
+  constructor(user) {
+    this.user = user;
+    this.auth = new UserAuth(user)
+  }
+  
+  changeSettings(settings) {
+    if (this.auth.verifyCredentials()) {
+      // ...
+    }
+  }
+}
+```
+**[⬆ back to top](#table-of-contents)**
+
 ### Prefer ES6 classes over ES5 plain functions
 It's very difficult to get readable class inheritance, construction, and method
 definitions for classical ES5 classes. If you need inheritance (and be aware