Nodejs Question about circular dependency-CodePudding

I was testing circular dependency with commonjs.

//index.js
console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

//a.js
console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

//b.js
console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

The result was

// output
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done = true, b.done = true

So index import a and a import b

b try to import a but a is not finished so a return unfinished module.

OK fine, I understood.

But what about esmodule syntax ?

I reproduced code like this.

//index.js

console.log('main starting');
import a from './a.js';
import b from './b.js';
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

//a.js
console.log('a starting');
export let done = false;
import {done as B_done}  from './b.js';
console.log('in a, b.done = %j', B_done);
done = true;
console.log('a done');
export default {
    done
}

//b.js
console.log('b starting');
export let done = false;
import {done as A_done} from './a.js';
console.log('in b, a.done = %j', A_done);
done = true;
console.log('b done');
export default {
    done
}

But the result was changed.

ReferenceError: Cannot access 'A_done' before initialization

This error occrued.

Why is it giving me a different result than commonjs ?

Doesn't esmodule return unfinished modules?

You can actually find this code sample in here

CodePudding user response：

Doesn't esmodule return unfinished modules?

It does. Try with var instead of let.

Why is it giving me a different result than commonjs?

Because it also changed evaluation order. Imports are "hoisted", the evaluation of the module code is deferred until all dependencies are set up (that is, either completely finished evaluating, or having their variables declared and waiting for circular dependencies). Unlike the require() call, you won't get the initialisation code of b executing in the middle of a.