如何为传递给构造函数的对象设置默认属性？

Question

我试图在一处进行更改以影响传递给对象的所有实例化的配置对象。 该对象在全局范围内可用，如下所示：

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

在我的项目中使用var myFoo = new Crayons().foo({color: "red"});初始化该对象var myFoo = new Crayons().foo({color: "red"});

我想将{color: "blue"}为默认值，这样，如果某人没有通过颜色，则设置为蓝色。

我试着做

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。

我想念什么？

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
  }
}

或者只是将其设为返回实例的工厂：

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
  }
}

在这里，您还可以在调用var myFoo = Crayons.foo({color: "red"});时删除new var myFoo = Crayons.foo({color: "red"});

创建实例后修改实例的另一种方法是装饰传入的选项，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);