为什么在这里一般调用hasOwnProperty？ [英] Why is hasOwnProperty being invoked generically here?

问题描述

Axel Rauschmayer的 Speaking Javascript：深入的程序员指南中提到了以下功能：

The following function is mentioned in Speaking Javascript: An In-Depth Guide for Programmers by Axel Rauschmayer:

function getDefiningObject(obj, propKey) {
    obj = Object(obj); // make sure it’s an object
    while (obj && !{}.hasOwnProperty.call(obj, propKey)) {
        obj = Object.getPrototypeOf(obj);
        // obj is null if we have reached the end
    }
    return obj;
}

正如作者所说，它的目的是迭代对象的属性链 obj [并返回]具有自己的属性且键为 propKey 的第一个对象，或者 null 如果没有这样的对象。

Its purpose, as the author puts it, is "to [iterate] over the property chain of an object obj [and return] the first object that has an own property with the key propKey, or null if there is no such object".

我理解这里的整体推理，但我不知道理解为什么 {}。hasOwnProperty.call（obj，propKey）正在完成而不仅仅是 obj.hasOwnProperty（propKey）。任何想法？

I understand the overall reasoning here, but what I don't understand is why {}.hasOwnProperty.call(obj, propKey) is being done rather than just obj.hasOwnProperty(propKey). Any ideas?

推荐答案

通常情况下，开发人员会在构建时使用调用 -in类型以确保它们获得方法的正确本机行为而不是某些被覆盖的行为。

这是一项我们真正不应该采取的保护措施使用，但因为对象在JavaScript中是如此具有可塑性，它保证我们得到我们想要的行为。

Often, developers will use call on a built-in type to ensure that they are getting the correct native behavior of a method and not some overridden behavior.

It is a protective measure that we really shouldn't have to use, but because Objects are so malleable in JavaScript, it guarantees we get the behavior we desire.

想象一下，如果有人（有意或无意地）创建了这样的对象：

Imagine if someone (who, intentionally or accidentally) created an object like this:

function SuperObject(){
   this.foo = function(){
     // Something here
   };

   this.hasOwnProperty = 42;
}

然后，你来了（没看过对象的实现）并写道：

And then, you came along (without having seen the implementation of the object) and wrote:

var mySuperObject = new SuperObject();
console.log("Does 'mySuperObject' instance have its own 'foo' property? " +
            mySuperObject.hasOwnProperty("foo"));

你会得到这个：

function SuperObject(){
  this.foo = function(){
    // Something here
  };
    
  this.hasOwnProperty = 42;
}


var mySuperObject = new SuperObject();

// Safe way:
console.log("Does 'mySuperObject' instance have its own 'foo' property? " +
             {}.hasOwnProperty.call(mySuperObject, "foo"));

// Unsafe way (fails):
console.log("Does 'mySuperObject' instance have its own 'foo' property? " +
             mySuperObject.hasOwnProperty("foo"));

所以，我们得到这样的代码：

So, instead we get code like this:

// This creates a new "fresh" (and unaltered) object with "object literal"
// syntax that you know doesn't override "hasOwnProperty"
{}.hasOwnProperty.call(obj, propKey)

// This performs the same task, but uses the native prototype object
// of the built-in Object type that we also know hasn't been altered:
Object.prototype.hasOwnProperty.call(obj, propKey) 

它强制在na的原型链上查找 hasOwnProperty tive 对象，同时：

It forces the hasOwnProperty to be looked up on the prototype chain of the native Object, while:

obj.hasOwnProperty(propKey)

依赖于该对象的特定实例上可用且正确的属性（ obj ）。

relies on that property being available and correct on a particular instance of an object (obj).

其他受欢迎的例子是Array类型：

Other popular examples of this are with the Array type:

// These work the same as the Object examples above, but just 
// do it for the Array type:
[].forEach.call(); // Array literal syntax
Array.prototype.forEach.call(); // Explicit array syntax

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