Answer "What are the differences between ES6 class and ES5 function c…

…onstructors?" (yangshun#104)

questions/javascript-questions.md

@@ -970,7 +970,54 @@ baz = 'qux';
 
 ### What are the differences between ES6 class and ES5 function constructors?
 
-TODO
+Let's first look at example of each:
+
+```js
+// ES5 Function Constructor
+function Person(name) {
+  this.name = name;
+}
+
+// ES6 Class
+class Person {
+  constructor(name) {
+    this.name = name;
+  }
+}
+```
+
+For simple constructors, they look pretty similar.
+
+The main difference in the constructor comes when using inheritance. If we want to create a `Student` class that subclasses `Person` and add a `studentId` field, this is what we have to do in addition to the above.
+
+```js
+// ES5 Function Constructor
+function Student(name, studentId) {
+  // Call constructor of superclass to initialize superclass-derived members.
+  Person.call(this, name);
+
+  // Initialize subclass's own members.
+  this.studentId = studentId;
+}
+
+Student.prototype = Object.create(Person.prototype);
+Student.prototype.constructor = Student;
+
+// ES6 Class
+class Student extends Person {
+  constructor(name, studentId) {
+    super(name);
+    this.studentId = studentId;
+  }
+}
+```
+
+It's much more verbose to use inheritance in ES5 and the ES6 version is easier to understand and remember.
+
+###### References
+
+* https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Inheritance
+* https://eli.thegreenplace.net/2013/10/22/classical-inheritance-in-javascript-es5
 
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