Javascript ES6 export const vs export let [英] Javascript ES6 export const vs export let

问题描述

假设我有一个我要导出的变量。

  export const a = 1; 
  

vs

  export let a = 1; 
  

我了解 const 和 let ，但是当您输出它们时，有什么区别？

解决方案

在ES6中， import 是导出值的实时只读视图。因此，当您执行从somemodule导入a; 时，无论如何，您不能分配到 a 在模块中声明 a 。

但是，由于导入的变量是直播视图，他们根据出口的原始出口变量进行变化。请考虑以下代码（从以下参考文献中借用）：

  // ------ lib.js  - ---- 
 export let counter = 3; 
 export function incCounter（）{
 counter ++; 
} 
 
 // ------ main1.js ------ 
 import {counter，incCounter} from'./lib'; 
 
 //导入的值`counter`是live 
 console.log（counter）; // 3 
 incCounter（）; 
 console.log（counter）; // 4 
 
 //导入的值不能被更改
 counter ++; // TypeError 
  

正如你所看到的，区别在于 lib。 js ，而不是 main1.js 。

---

总结：

• 您无法分配到 import -ed变量，无论你如何在模块中声明相应的变量。


• 传统的让 -vs- const 语义适用于模块中声明的变量。
  
  
  
  
  • 如果变量声明为 const ，则无法在任何地方重新分配或反弹。 >
    
  • 如果变量声明为 let ，则只能在模块中重新分配（而不是用户）。如果更改， import -ed变量相应地更改。
  
  

参考：
http://exploringjs.com/es6/ch_modules.html#leanpub-auto-in-es6-imports-are-live-read-only-views-on-exported价值

Let's say I have a variable that I want to export. What's the difference between

export const a = 1;

vs

export let a = 1;

I understand the difference between const and let, but when you export them, what are the differences?

解决方案

In ES6, imports are live read-only views on exported-values. As a result, when you do import a from "somemodule";, you cannot assign to a no matter how you declare a in the module.

However, since imported variables are live views, they do change according to the "raw" exported variable in exports. Consider the following code (borrowed from the reference article below):

//------ lib.js ------
export let counter = 3;
export function incCounter() {
    counter++;
}

//------ main1.js ------
import { counter, incCounter } from './lib';

// The imported value `counter` is live
console.log(counter); // 3
incCounter();
console.log(counter); // 4

// The imported value can’t be changed
counter++; // TypeError

As you can see, the difference really lies in lib.js, not main1.js.

---

To summarize:

• You cannot assign to import-ed variables, no matter how you declare the corresponding variables in the module.
• The traditional let-vs-const semantics applies to the declared variable in the module.
  • If the variable is declared const, it cannot be reassigned or rebound in anywhere.
  • If the variable is declared let, it can only be reassigned in the module (but not the user). If it is changed, the import-ed variable changes accordingly.

Reference: http://exploringjs.com/es6/ch_modules.html#leanpub-auto-in-es6-imports-are-live-read-only-views-on-exported-values

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