实现组合继承

/**

• Person
• @param {String} name
  */
  function Person(name) {
  this.name = name;
  }
  Person.prototype.sayHello = function() {
  console.log('Hi! My name is ' + this.name + '.');
  }

// TODOS: 改写 Student 类，接收 name 和 grade 属性，并使其继承 Person, 创建实例既有 name 实例属性，又有 sayHello 原型方法。
function Student(name,grade) {
this.grade = grade;
Person.call(this,name);
}
Student.prototype = new Person();
Student.prototype.constructor = Student;
Student.prototype.sayGrade = function() {
console.log('I am Grade ' + this.grade + '.');
}

// 可以正确运行下面代码：
var student = new Student('Cover', 4);
console.log(student.name); // 'Cover'
student.sayHello(); // 'Hi! My name is Cover.'
student.sayGrade(); // 'I am Grade 4.'
student.hasOwnProperty('name'); // true