《JavaScript应用程序设计》一一3.7 创建对象

本节书摘来华章计算机出版社《JavaScript应用程序设计》一书中的第3章，第3.7节，作者：Eric Elliott 更多章节内容可以访问云栖社区“华章计算机”公众号查看。

3.7 创建对象

JavaScript中的对象一部分是通过构造函数生成的，例如：

function Car(color, direction, mph) {
  this.color = color || 'pink';
  this.direction = direction || 0; // 0 = Straight ahead
  this.mph = mph || 0;

  this.gas = function gas(amount) {
    amount = amount || 10;
    this.mph += amount;
    return this;
  };

  this.brake = function brake(amount) {
    amount = amount || 10;
    this.mph = ((this.mph - amount) < 0)? 0
      : this.mph - amount;
    return this;
  };
}

var myCar = new Car();

test('Constructor', function () {
  ok(myCar.color, 'Has a color');

  equal(myCar.gas().mph, 10,
    '.accelerate() should add 10mph.'
  );

  equal(myCar.brake(5).mph, 5,
    '.brake(5) should subtract 5mph.'
  );      
});

使用私有变量来做数据封装：

function Car(color, direction, mph) {
  var isParkingBrakeOn = false;
  this.color = color || 'pink';
  this.direction = direction || 0; // 0 = Straight ahead
  this.mph = mph || 0;

  this.gas = function gas(amount) {
    amount = amount || 10;
    this.mph += amount;
    return this;
  };
  this.brake = function brake(amount) {
    amount = amount || 10;
    this.mph = ((this.mph - amount) < 0)? 0
      : this.mph - amount;
    return this;
  };
  this.toggleParkingBrake = function toggleParkingBrake() {
    isParkingBrakeOn = !isParkingBreakOn;
    return this;
  };
  this.isParked = function isParked() {
    return isParkingBrakeOn;
  }
}

var myCar = new Car();

test('Constructor with private property.', function () {
  ok(myCar.color, 'Has a color');
  equal(myCar.gas().mph, 10,
    '.accelerate() should add 10mph.'
  );
  equal(myCar.brake(5).mph, 5,
    '.brake(5) should subtract 5mph.'
  );
  ok(myCar.toggleParkingBrake().isParked(),
    '.toggleParkingBrake works.'
  );
});

对象字面量让对象构建的语法更为简单直观：

var myCar = {
  color: 'pink',
  direction: 0,
  mph: 0,

  gas: function gas(amount) {
    amount = amount || 10;
    this.mph += amount;
    return this;
  },

  brake: function brake(amount) {
    amount = amount || 10;
    this.mph = ((this.mph - amount) < 0)? 0
      : this.mph - amount;
    return this;
  }
};

test('Object literal notation.', function () {
  ok(myCar.color, 'Has a color');

  equal(myCar.gas().mph, 10,
    '.accelerate() should add 10mph.'
  );

  equal(myCar.brake(5).mph, 5,
    '.brake(5) should subtract 5mph.'
  );
});

请注意，由于对象字面量不具备函数作用域，所以数据封装性会丢失。