哪种方式最适合在 JavaScript 中创建对象?在对象属性之前是否需要 `var`? [英] Which way is best for creating an object in JavaScript? Is `var` necessary before an object property?

问题描述

到目前为止，我看到了在 JavaScript 中创建对象的三种方法.哪种方式最适合创建对象?为什么?

So far I saw three ways for creating an object in JavaScript. Which way is best for creating an object and why?

我还看到在所有这些示例中，关键字 var 没有在属性之前使用 — 为什么?var 不是必须在属性名称前声明，因为它提到属性是变量吗?

I also saw that in all of these examples the keyword var is not used before a property — why? Is it not necessary to declare var before the name of a property as it mentioned that properties are variables?

在第二种和第三种方式中，对象的名称是大写的，而在第一种方式中，对象的名称是小写的.对象名称应该使用什么大小写?

In the second and third way, the name of the object is in upper-case whereas in the first way the name of the object is in lower-case. What case should we use for an object name?

function person(fname, lname, age, eyecolor){
  this.firstname = fname;
  this.lastname = lname;
  this.age = age;
  this.eyecolor = eyecolor;
}

myFather = new person("John", "Doe", 50, "blue");
document.write(myFather.firstname + " is " + myFather.age + " years old.");

第二种方式:

var Robot = {
  metal: "Titanium",
  killAllHumans: function(){
    alert("Exterminate!");
  }
};

Robot.killAllHumans();

第三种方式 — 使用数组语法的JavaScript对象:

var NewObject = {};

NewObject['property1'] = value;
NewObject['property2'] = value;
NewObject['method'] = function(){ /* function code here */ }

推荐答案

没有最佳方法，这取决于您的用例.

There is no best way, it depends on your use case.

• 如果您想创建多个相似的对象，请使用方式 1.在您的示例中，Person(名称应以大写字母开头)称为构造函数.这类似于其他面向对象语言中的类.
• 如果您只需要一个对象(如单例)，请使用方式 2.如果您希望此对象从另一个对象继承，那么您必须使用构造函数.
• 如果您想根据对象的其他属性初始化对象的属性，或者您有动态属性名称，请使用方式 3.

• Use way 1 if you want to create several similar objects. In your example, Person (you should start the name with a capital letter) is called the constructor function. This is similar to classes in other OO languages.
• Use way 2 if you only need one object of a kind (like a singleton). If you want this object to inherit from another one, then you have to use a constructor function though.
• Use way 3 if you want to initialize properties of the object depending on other properties of it or if you have dynamic property names.

更新:作为第三种方式的请求示例.

Update: As requested examples for the third way.

依赖属性:

以下内容不起作用，因为this不引用book.无法使用对象字面量中的其他属性的值来初始化属性:

The following does not work as this does not refer to book. There is no way to initialize a property with values of other properties in a object literal:

var book = {
    price: somePrice * discount,
    pages: 500,
    pricePerPage: this.price / this.pages
};

相反，你可以这样做:

var book = {
    price: somePrice * discount,
    pages: 500
};
book.pricePerPage = book.price / book.pages;
// or book['pricePerPage'] = book.price / book.pages;

动态属性名称:

如果属性名称存储在某个变量中或通过某个表达式创建，则必须使用括号表示法:

If the property name is stored in some variable or created through some expression, then you have to use bracket notation:

var name = 'propertyName';

// the property will be `name`, not `propertyName`
var obj = {
    name: 42
}; 

// same here
obj.name = 42;

// this works, it will set `propertyName`
obj[name] = 42;

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