Interface Segregation Principle (ISP)

Author: ryanmcdermott

README.md

@@ -999,6 +999,80 @@ renderLargeShapes();
 ```
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+### Interface Segregation Principle (ISP)
+JavaScript doesn't have interfaces so this principle doesn't apply as strictly
+as others. However, it's important and relevant even with JavaScript's lack of
+type system.
+
+ISP states that "Clients should not be forced to depend upon interfaces that
+they do not use." Interfaces are implicit contracts in JavaScript because of
+duck typing.
+
+A good example to look at that demonstrates this principle in JavaScript is for
+classes that require large settings objects. Not requiring clients to setup
+huge amounts of options is beneficial, because most of the time they won't need
+all of the settings. Making them optional helps prevent having a "fat interface".
+
+**Bad:**
+```javascript
+class DOMTraverser {
+  constructor(settings) {
+    this.settings = settings;
+    this.setup();
+  }
+
+  setup() {
+    this.rootNode = this.settings.rootNode;
+    this.animationModule.setup();
+  }
+
+  traverse() {
+    // ...
+  }
+}
+
+let $ = new DOMTraverser({
+  rootNode: document.getElementsByTagName('body'),
+  animationModule: function() {} // Most of the time, we won't need to animate when traversing.
+  // ...
+});
+
+```
+
+**Good**:
+```javascript
+class DOMTraverser {
+  constructor(settings) {
+    this.settings = settings;
+    this.options = settings.options;
+    this.setup();
+  }
+
+  setup() {
+    this.rootNode = this.settings.rootNode;
+    this.setupOptions();
+  }
+
+  setupOptions() {
+    if (this.options.animationModule) {
+      // ...
+    }
+  }
+
+  traverse() {
+    // ...
+  }
+}
+
+let $ = new DOMTraverser({
+  rootNode: document.getElementsByTagName('body'),
+  options: {  
+    animationModule: function() {}
+  }
+});
+```
+**[⬆ 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