Javascript ES6，为什么我不能使用带有箭头功能的`new`? [英] Javascript ES6, why I can not use `new` with arrow function?

问题描述

据我所知，箭头函数类似于普通函数.我这样使用是没有问题的:

let X = () =>{};让 Y = 函数(){}；X();Y();

但是，当我将它们与 new

一起使用时发生错误

let X = () =>{};让 Y = 函数(){}；x = 新的 X();y = new Y();

未捕获的类型错误:X 不是构造函数

你能解释一下为什么吗?非常感谢.

解决方案

您可能希望澄清您的问题.

问我做错了什么?

A. 您使用了带有箭头功能的 new，这是不允许的.

问:我可以将箭头函数变成构造函数吗?

A. 仅将其包装在普通函数中，这很愚蠢.你不能把箭头函数本身变成构造函数.

问你能解释一下规范是如何禁止带有箭头函数的new的吗?

A. 要成为构造函数，函数对象必须具有a [[Construct]]内部方法.

使用 function 创建的函数关键字是构造函数，一些内置函数也是例如Date.这些是您可以与 new 一起使用的函数.

其他函数对象没有[[Construct]]内部方法.这些包括箭头函数.所以你不能将 new 与这些一起使用.这是有道理的，因为您 可以没有设置箭头函数的 this 值.

一些内置函数也不是构造函数.例如.你不能做 new parseInt().

Q. 你能解释一下禁止 new 背后的原因吗?规范中有箭头函数吗?

A. 使用常识，或搜索 es-discuss 档案.>

As far as I know, arrow function is similar to normal function. There is no problem when I use like this:

let X = () => {};
let Y = function() {};
X();
Y();

However, error occurred when I use them with new

let X = () => {};
let Y = function() {};
x = new X();
y = new Y();

Uncaught TypeError: X is not a constructor

Would you please explain me why? Many thanks.

解决方案

You may wish to clarify your question.

Q. What did I do wrong?

A. You used new with an arrow function, and that's not allowed.

Q. Can I turn an arrow function into a constructor?

A. Only by wrapping it in a normal function, which would be silly. You can't turn an arrow function itself into a constructor.

Q. Can you explain how the specification disallows new with arrow functions?

A. To be a constructor, a function object must have a [[Construct]] internal method.

Functions created with the function keyword are constructors, as are some built-in functions such as Date. These are the functions you can use with new.

Other function objects do not have a [[Construct]] internal method. These include arrow functions. So you can't use new with these. This makes sense since you can't set the this value of an arrow function.

Some built-in functions are also not constructors. E.g. you can't do new parseInt().

Q. Can you explain the rationale behind disallowing new with arrow functions in the specification?

A. Use common sense, or search the es-discuss archives.

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