面向对象的JavaScript-001

一、

Question是父类，MultipleChoiceQuestion和DragDropQuestion是子类

二、

1.

 <script>
 // 面向对象
     function Question(theQuestion, theChoices, theCorrectAnswer) {
         // Initialize the instance properties​
         this.question = theQuestion;
         this.choices = theChoices;
         this.correctAnswer = theCorrectAnswer;
         this.userAnswer = "";
         // private properties: these cannot be changed by instances​
         var newDate = new Date(),
         // Constant variable: available to all instances through the instance method below. This is also a private property.​
         QUIZ_CREATED_DATE = newDate.toLocaleDateString();
         // This is the only way to access the private QUIZ_CREATED_DATE variable ​
         // This is an example of a privilege method: it can access private properties and it can be called publicly​
         this.getQuizDate = function () {
             return QUIZ_CREATED_DATE;
         };
         // A confirmation message that the question was created​
         console.log("Quiz Created On: " + this.getQuizDate());
     }

     //Add Prototype Methods to The Question Object
      // Define the prototype methods that will be inherited​
     Question.prototype.getCorrectAnswer = function () {
         return  this.correctAnswer;
     };

     Question.prototype.getUserAnswer = function () {
         return this.userAnswer;
     };

     Question.prototype.displayQuestion = function () {
         var questionToDisplay = "<div class='question'>" + this.question + "</div><ul>";
             choiceCounter = 0;
             this.choices.forEach(function (eachChoice)  {
             questionToDisplay += '<li><input type="radio" name="choice" value="' + choiceCounter + '">' + eachChoice + '</li>';
             choiceCounter++;
         });
         questionToDisplay += "</ul>";

         console.log (questionToDisplay);
     }; 

     function inheritPrototype(childObject, parentObject) {
        // As discussed above, we use the Crockford’s method to copy the properties and methods from the parentObject onto the childObject​
     // So the copyOfParent object now has everything the parentObject has ​
         var copyOfParent = Object.create(parentObject.prototype);

         //Then we set the constructor of this new object to point to the childObject.​
     // Why do we manually set the copyOfParent constructor here, see the explanation immediately following this code block.​
         copyOfParent.constructor = childObject;

         // Then we set the childObject prototype to copyOfParent, so that the childObject can in turn inherit everything from copyOfParent (from parentObject)​
        childObject.prototype = copyOfParent;
     }
     // Child Questions (Sub Classes of the Question object)
     // First, a Multiple Choice Question:
     // Create the MultipleChoiceQuestion​
     function MultipleChoiceQuestion(theQuestion, theChoices, theCorrectAnswer){
     // For MultipleChoiceQuestion to properly inherit from Question, here inside the MultipleChoiceQuestion constructor, we have to explicitly call the Question constructor​
     // passing MultipleChoiceQuestion as the this object, and the parameters we want to use in the Question constructor:​
         Question.call(this, theQuestion, theChoices, theCorrectAnswer);
     };
     // inherit the methods and properties from Question​
     inheritPrototype(MultipleChoiceQuestion, Question);

     // A Drag and Drop Question
     // Create the DragDropQuestion​
     function DragDropQuestion(theQuestion, theChoices, theCorrectAnswer) {
         Question.call(this, theQuestion, theChoices, theCorrectAnswer);
     }

     // inherit the methods and properties from Question​
     inheritPrototype(DragDropQuestion, Question);

     // Overriding Methods
     DragDropQuestion.prototype.displayQuestion = function () {
         // Just return the question. Drag and Drop implementation detail is beyond this article​
         console.log(this.question);
     };

     // Initialize some questions and add them to an array​
     var allQuestions = [
     new MultipleChoiceQuestion("Who is Prime Minister of England?", ["Obama", "Blair", "Brown", "Cameron"], 3),

     new MultipleChoiceQuestion("What is the Capital of Brazil?", ["São Paulo", "Rio de Janeiro", "Brasília"], 2),

     new DragDropQuestion("Drag the correct City to the world map.", ["Washington, DC", "Rio de Janeiro", "Stockholm"], 0)
     ];

     // Display all the questions​
     allQuestions.forEach(function (eachQuestion)  {
         eachQuestion.displayQuestion();
     });
 </script>