Javascript ES6 export const vs export let
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Javascript ES6 export const vs export let
提问by Cheng
Let's say I have a variable that I want to export. What's the difference between
假设我有一个要导出的变量。有什么区别
export const a = 1;
vs
对比
export let a = 1;
I understand the difference between constand let, but when you export them, what are the differences?
我明白之间的差别const和let，但是当你导出它们，有什么区别？
回答by FelisCatus
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 ano matter how you declare ain the module.
在 ES6 中，imports 是导出值的实时只读视图。因此，当您这样做时import a from "somemodule";，a无论您a在模块中如何声明，都无法分配给。
However, since imported variables are liveviews, they do change according to the "raw" exported variable in exports. Consider the following code (borrowed from the reference article below):
但是，由于导入的变量是实时视图，它们确实会根据导出中的“原始”导出变量而变化。考虑以下代码（从下面的参考文章中借用）：
//------ 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.
如您所见，区别真正在于lib.js，而不是main1.js。
To summarize:
总结一下：
- You cannot assign to
import-ed variables, no matter how you declare the corresponding variables in the module. - The traditional
let-vs-constsemantics 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, theimport-ed variable changes accordingly.
- If the variable is declared
import无论您如何在模块中声明相应的变量，都不能分配给-ed 变量。- 传统的
let-vs-const语义适用于模块中声明的变量。- 如果声明了变量
const，则不能在任何地方重新分配或重新分配。 - 如果声明了变量
let，则只能在模块中重新分配（但不能在用户中重新分配）。如果更改，import-ed 变量会相应更改。
- 如果声明了变量
参考：http: //exploringjs.com/es6/ch_modules.html#leanpub-auto-in-es6-imports-are-live-read-only-views-on-exported-values
回答by slomek
I think that once you've imported it, the behaviour is the same (in the place your variable will be used outside source file).
我认为一旦你导入它，行为是一样的（你的变量将在源文件之外使用）。
The only difference would be if you try to reassign it before the end of this very file.
唯一的区别是如果您尝试在此文件结束之前重新分配它。
