意外的令牌“导出”

Question

I am trying to export variables from index.js to profile.js. When I export at the top of the file such as

export let k = 12;

Or

export { k }; let k = 12;

It works just fine. However, when I export after the DOMContentLoaded event such as

document.addEventListener('DOMContentLoaded', function(){ export let k = 12; })

then it shows Unexpected token 'export' . The one bellow is also gives the same error

if (true){ export let k = 2; }

What did I do wrong here?

Answer 1

What should I do to be able to export variable after DOMContentLoaded

You can't. export and import can only appear at the top level of a module.

What you can do instead is export the variable, and set its value in the handler:

export let k;
document.addEventListener('DOMContentLoaded', function(){ k = 12; })

If other modules import k , they get a read-only binding to it, meaning that if they use it before DOMContentLoaded , they'll get whatever initial value you provide ( undefined in the above, because there's no initializer on the let k; declaration), but if they use it after the DOMContentLoaded , they'll see the value assigned by the handler.

That said, if your module provides information that is only available after a certain point, you might consider exporting a promise, like so:

let kPromiseResolve;
export const kPromise = new Promise(resolve => {
    kPromiseResolve = resolve;
});
document.addEventListener('DOMContentLoaded', function(){ kPromiseResolve(12); })

Then modules using it would do:

import { kPromise } from "./your-module.js";
kPromise
.then(k => {
    //  ...use `k`...
})
.catch(error => {
    // ...
});

or in environments supporting top-level await , if the importing module can't do anything useful without k , they could do:

import { kPromise } from "./your-module.js";
const k = await kPromise;
// ...use `k`...