在Javascript中声明函数的最有效方法是什么？ [英] What is the most efficient way to declare functions in Javascript?

问题描述

  function myfunction（fruit）{ 
 alert（'我喜欢'+水果+'！'）; 
} 
  

或类似物：

  var myfunction = function（水果）{
 alert（'我喜欢'+水果+'！'）; 
}; 
  

然而，最近我注意到有些人实际上把函数定义为常量：

  const myfunction = fruit =>警报（'我喜欢'+水果+'！'）; 
  

甚至使用关键字 let ：

  let myfunction = fruit =>警报（'我喜欢'+水果+'！'）; 
  

此时我很困惑。

• 为什么有这么多的定义函数的方法？


• 何时/应该在哪里使用每一个？


• 是否更有效？
我认为这取决于您的需求。例如




这将在你的本地范围内使用 myfunction 名称定义你的函数

  function myfunction（fruit）{
 alert（'我喜欢'+ fruit +'！'）; 
} 
  

另一方面，下面的代码将定义一个变量称为 myfunction ，它指向您本地作用域内的通用函数。



  var myfunction = function（fruit）{
 alert（'我喜欢'+ fruit +'！'）; 
}; 
  

下面的代码将定义一个 ECMA6的箭头功能，目前所有浏览器都不支持。此外， let 语句声明一个块作用域局部变量，可以将其初始化为一个值。因此，在代码块关闭后，您的 myfunction 变量将不会被看到。



  let myfunction = fruit =>警报（'我喜欢'+水果+'！'）; 
  

让允许您声明变量在范围上限于使用它的区块，声明或表达。您可以阅读更多内容并在此处查看一些示例



正如官方文件所言：






const声明创建一个read-只引用一个值。它
并不意味着它保存的值是不可变的，只是变量
标识符不能被重新分配。








  const myfunction = fruit =>警报（'我喜欢'+水果+'！'）; 
  

因此，如果您尝试重新分配 myfunction 会失败（默默地）（但在Safari中不会失败）

  //这将在Firefox和Chrome中无提示失败
 myfunction = fruit =>警报（'不！我不喜欢'+水果+'！'）; 
  

关于让和 const >相似之处MDN引用说明




常量是块范围的，就像使用let
语句。一个常量的值不能通过
重新赋值而改变，并且不能重新声明。




所以，as Aurelio de Rosa says ，






常量与使用let声明的变量共享一个特性
它们是块范围的而不是函数范围的


阅读更多关于 const 这里




I have always learned that to declare a function in javascript you should do something like:

function myfunction(fruit){
    alert('I like ' + fruit + '!');
}

or something like:

var myfunction = function(fruit){
    alert('I like ' + fruit + '!');
};

However, most recently, I have noticed that some people actually define functions as constants:

const myfunction = fruit=> alert('I like ' + fruit + '!');

Or even using the keyword let:

let myfunction = fruit=> alert('I like ' + fruit + '!');

At this point I am quite confused.

• Why so many ways of defining functions?
• When/where should I use each one?
• Which way is more efficient?

解决方案

I think it will depend on your needs. For example

this will define your function with myfunction name on your local scope

function myfunction(fruit){
    alert('I like ' + fruit + '!');
}

on the other hand, the code below will define a variable called myfunction that points to an annonimous function inside your local scope.

var myfunction = function(fruit){
    alert('I like ' + fruit + '!');
};

while the code below will define an arrow function of ECMA6 that are not supported by all browsers at the current date. Besides, let statement declares a block scope local variable, optionally initializing it to a value. So your myfunction variable won't be seen after the code block is closed.

let myfunction = fruit=> alert('I like ' + fruit + '!');

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. You can read more and see some examples here

As the oficial documentation says:

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.

const myfunction = fruit=> alert('I like ' + fruit + '!');

So if you try to reassign myfunction it will fail (silently) (but does not fail in Safari)

// this will fail silently in Firefox and Chrome 
myfunction = fruit=> alert('No! I DO NOT like ' + fruit + '!');

About let and const similarities the MDN reference says that

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

So, as Aurelio de Rosa says,

constants share a feature with variables declared using let in that they are block-scoped instead of function-scoped

Read more about const here

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