ecmascript 6 - Javascript ES6 export const vs export let

Question

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?

Answer 1

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