如何为传递给构造函数的对象设置默认值属性？

Question

我试图在一个地方进行更改以影响传递到对象的所有实例化的配置对象。该对象在全球范围内可用，如下所示：

function Crayons(){
  return {
    foo: ThirdPartyFoo
  }
}

对象在我的项目中用 var myFoo = new Crayons().foo({color: "red"});

初始化

我想将 {color: "blue"} 设置为默认值，这样如果有人不输入颜色，就会设置为蓝色。

我试过

function Crayons(){
  var fooWithDefaults = function(){
    this = new ThirdPartyFoo(arguments); //this is invalid
    this.color = "blue"; //and this would overwrite color if it was set
  }

  return {
    foo: fooWithDefaults
  }
}

但是 new 关键字让我失望，因为我不知道如何创建本质上说 this = new 3rdPartyFoo.

的 javascript 构造函数

我错过了什么？

Answer 1

您可以装饰构造函数：

function Crayons(){
  function fooWithDefaults() {
    3rdPartyFoo.apply(this, arguments); // this is what you're looking for
    if (!this.color) // or whatever to detect "not set"
      this.color = "blue";
  }
  fooWithDefaults.prototype = 3rdPartyFoo.prototype; // to make `new` work

  return {
    foo: fooWithDefaults
  }
}

或者你只是让它成为一个 returns 实例的工厂：

function Crayons(){
  function fooWithDefaults(arg) {
    var that = new 3rdPartyFoo(arg); // you should know how many arguments it takes
    if (!that.color) // or whatever to detect "not set"
      that.color = "blue";
    return that;
  }

  return {
    foo: fooWithDefaults
  }
}

这里你也可以在调用var myFoo = Crayons.foo({color: "red"});

时去掉new

创建实例后修改实例的替代方法是修饰传入的选项，这通常是更好的解决方案：

function Crayons(){
  function fooWithDefaults(arg) {
    if (!arg.color) // or whatever to detect "not set"
      arg.color = "blue";
    return new 3rdPartyFoo(arg);
  }

  return {
    foo: fooWithDefaults
  }
}

Answer 2

function ThirdPartyCrayon(config) {           // constructor :: possible original Crayon implementation

    Object.assign(this, config);
    this.type = "thirdpartycrayon";
}


function createCrayonSetting(defaultConfig) { // factory (creates a closure)

    function CrayonWithDefaults() {           // constructor :: customized Crayon wrapper

        Object.assign(this, defaultConfig);
        ThirdPartyCrayon.apply(this, arguments);
    }

    return {
        Crayon: CrayonWithDefaults
    }
}


var
    setting = createCrayonSetting({color: "blue", strokeWidth: "thin"}),

    crayon1 = new setting.Crayon({color: "red", strokeWidth: "bold"}),
    crayon2 = new setting.Crayon({color: "green"});
    crayon3 = new setting.Crayon();


console.log("crayon1 : ", crayon1);
console.log("crayon2 : ", crayon2);
console.log("crayon3 : ", crayon3);

如何为传递给构造函数的对象设置默认值属性？

How can I set a default property for an object that's passed into a constructor?

javascript

constructor

factory

wrapper

javascript-objects

如何为传递给构造函数的对象设置默认值 属性？

How can I set a default property for an object that's passed into a constructor?

javascript

constructor

factory

wrapper

javascript-objects

如何为传递给构造函数的对象设置默认值属性？