javascript new关键字对于javascript API功能是否可选？ [英] Is javascript new keyword optional for javascript API feature?

问题描述

我很难理解为什么 new 关键字对JavaScript API对象/接口功能有利。

I having trouble to understand why the new keyword is facultative for javascript API object/interface feature.

d = new String(); // javascript native object
d2 = String();
console.log(d);
console.log(d2);

控制台中的结果（这看起来很正常）：

results in console (that seems pretty normal):

 String {}
 (an empty string)

但是：

b = new Blob(); // API object
b2 = Blob();
console.log(b);
console.log(b2);

结果：

Blob { size=0, constructor=function(), type="", more...}
Blob { size=0, constructor=function(), type="", more...}

而不是：

Blob { size=0, constructor=function(), type="", more...}
Blob() is undefined or (an empty blob)

一切正常，但我很好奇...

It all work just fine but I'm curious...

推荐答案

这是JS中的疣。有一些内部构造对象并返回它们的函数（不需要 new ），还有一些使用 new 做一堆魔术。

It's a wart in JS. There are functions that construct objects internally and return them (don't need new), and "constructor functions" using new that do a bunch of magic.

通常的建议是永远不要编写自己的类型来要求 new ，因为 new 要求功能的确做得不好，但如果您忽略它，就会有些微妙的事情（具体来说，它们将 this 绑定到全局对象而不是新对象，因此实例变量实际上是全局变量，并为实例成员分配全局变量）。如果必须使用它们，JavaScript：The Good Parts建议在名称中使用首字母大写来表示 require new和其他所有字母的首字母小写。

The usual recommendation is to never write your own types to require new, because new requiring functions do bad but subtle things if you omit it (specifically, they bind this to the global object instead of a new object, so the instance variables are really the global variables, and assigning to instance members clobbers globals). If you must use them, JavaScript: The Good Parts recommends using initial capitals in the name to mean "requires new" and initial lowercase for everything else.

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