CODEWITHSUNDEEP
javascriptwebpackbabeljs
Alec Mev
Alec Mev

Reputation: 4813

Webpack import returns undefined, depending on the order of imports

I'm using webpack + babel. I have three modules looking like this:

// A.js

// some other imports here
console.log('A');
export default 'some-const';

// B.js

import someConst from './A';
console.log('B', someConst);
export default 'something-else';

// main.js

import someConst from './A';
import somethingElse from './B';
console.log('main', someConst);

When main.js is executed, I see the following:

B undefined
A
main some-const

If I swap the imports in main.js, B becoming the first, I get:

A
B some-const
main some-const

How come B.js gets undefined instead of a module in the first version? What's wrong?

Upvotes: 93

Views: 39975

Answers (3)

mseyfayi
mseyfayi

Reputation: 144

In my case problem was solved by changing output libraryTarget to commonjs2 in the webpack config:

  output: {
    path: `${__dirname}/dist`,
    filename: '[name].js',
    libraryTarget: 'commonjs2',
  },

Upvotes: 1

Raz
Raz

Reputation: 454

This plugin(pointed above) solved the issue for me as well.

I recommend setting this flag to avoid adding additional circular dependencies in the future.

// add errors to webpack instead of warnings
failOnError: true

There is also a eslint plugin to address it.

Upvotes: 1

Alec Mev
Alec Mev

Reputation: 4813

After almost a full workday of narrowing down the issue (AKA hair-pulling), I've finally came to realize that I have a circular dependency.

Where it says // some other imports here, A imports another module C, which, in turn, imports B. A gets imported first in main.js, so B ends up being the last link in the "circle", and Webpack (or any CommonJS-like environment, for that matter, like Node) just short-circuits it by returning A's module.exports, which is still undefined. Eventually, it becomes equal to some-const, but the synchronous code in B ends up dealing with undefined instead.

Eliminating the circular dependency, by moving out the code that C depends on out of B, has resolved the issue. Wish Webpack would somehow warn me about this.

Edit: On the last note, as pointed out by @cookie, there's a plugin for circular dependency detection, if you'd like to avoid hitting this problem [again].

Upvotes: 215

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