面向对象的JavaScript-002

1.

<script type="text/javascript">
    
    // Define the Person constructor
    var Person = function(firstName) {
      this.firstName = firstName;
    };

    // Making sure that this points to the right thing regardless of how the object is instantiated can be difficult. However, there is a simple idiom to make this easier.
    var Person = function(firstName) {
      if (this instanceof Person) {
        this.firstName = firstName;
      } else {
        return new Person(firstName);
      }
    }

    // Add a couple of methods to Person.prototype
    Person.prototype.walk = function(){
      console.log("I am walking!");
    };

    Person.prototype.sayHello = function(){
      console.log("Hello, I'm " + this.firstName);
    };

    // Define the Student constructor
    function Student(firstName, subject) {
      // Call the parent constructor, making sure (using Function#call)
      // that "this" is set correctly during the call
      Person.call(this, firstName);

      // Initialize our Student-specific properties
      this.subject = subject;
    }

    // Create a Student.prototype object that inherits from Person.prototype.
    // Note: A common error here is to use "new Person()" to create the
    // Student.prototype. That's incorrect for several reasons, not least 
    // that we don't have anything to give Person for the "firstName" 
    // argument. The correct place to call Person is above, where we call 
    // it from Student.
    Student.prototype = Object.create(Person.prototype); // See note below

    // Set the "constructor" property to refer to Student
    Student.prototype.constructor = Student;

    // Replace the "sayHello" method
    Student.prototype.sayHello = function(){
      console.log("Hello, I'm " + this.firstName + ". I'm studying "
                  + this.subject + ".");
    };

    // Add a "sayGoodBye" method
    Student.prototype.sayGoodBye = function(){
      console.log("Goodbye!");
    };

    // Example usage:
    var student1 = new Student("Janet", "Applied Physics");
    student1.sayHello();   // "Hello, I'm Janet. I'm studying Applied Physics."
    student1.walk();       // "I am walking!"
    student1.sayGoodBye(); // "Goodbye!"

    // Check that instanceof works correctly
    console.log(student1 instanceof Person);  // true 
    console.log(student1 instanceof Student); // true

    // 如果 On older JavaScript engines without Object.create, one can either use a "polyfill" (aka "shim", see the linked article), or one can use a function that achieves the same result, such as:
    function createObject(proto) {
        function ctor() { }
        ctor.prototype = proto;
        return new ctor();
    }

    // Usage:
    Student.prototype = createObject(Person.prototype);