“new <function>"后面的括号选修的?

问题描述

我在 Chrome 控制台中运行了以下一对代码片段，结果相同:

I have run the following pair of code snippets in Chrome console, with the same results:

test = new function(){
  var a = 1;
  var b = 2;
  var c = 3;
  this.debugBase = function(){console.log('' + a + b + c)};
};
test
debugBase: function (){console.log('' + a + b + c)}
__proto__: Object

对比:

test2 = new (function(){
  var a = 1;
  var b = 2;
  var c = 3;
  this.debugBase = function(){console.log('' + a + b + c)};
})();
test2
debugBase: function (){console.log('' + a + b + c)}
__proto__: Object

我错过了什么吗?函数后面的括号有什么意义吗?如果不是，为什么人们把它们放在那里?

Am I missing something? Is there any significance to the parentheses after the function? If not, why do people put them there?

推荐答案

这里问了一个类似的问题:new MyClass() 与 new MyClass.Daniel Vassallo 的完整且被接受的答案:

A similar questions was asked here: new MyClass() vs. new MyClass. The complete and accepted answer by Daniel Vassallo:

引用大卫弗拉纳根¹:

作为一种特殊情况，仅对于 new 运算符，JavaScript 通过在函数调用中没有参数时允许省略括号来简化语法.以下是一些使用 new 运算符的示例:

As a special case, for the new operator only, JavaScript simplifies the grammar by allowing the parenthesis to be omitted if there are no arguments in the function call. Here are some examples using the new operator:

o = new Object;  // Optional parenthesis omitted here
d = new Date();  

<小时>

就个人而言，我总是使用括号，即使构造函数不带参数.

Personally, I always use the parenthesis, even when the constructor takes no arguments.

另外，如果省略括号，JSLint 可能会伤害您的感受.它报告 Missing '()' invoking a constructor，并且该工具似乎没有允许括号省略的选项.

In addition, JSLint may hurt your feelings if you omit the parenthesis. It reports Missing '()' invoking a constructor, and there doesn't seem to be an option for the tool to tolerate parenthesis omission.

¹ David Flanagan:JavaScript 权威指南:第 4 版 (第 75 页)

¹ David Flanagan: JavaScript the Definitive Guide: 4th Edition (page 75)

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