Why so strange behaviour when using "require" in NodeJS?

Question

When I import ClassA inside ClassB, and then import ClassA inside ClassB, the imported ClassA is an empty Object. To demonstrate, I have these 3 files:

ClassA.js

const ClassB = require('./ClassB');

class ClassA {
    static test() {
        ClassB.test();
    }
}

module.exports = ClassA;

ClassB.js

const ClassA = require('./ClassA');

class ClassB {
    static test() {
        console.log('ClassA.test', ClassA.test);
    }
}

module.exports = ClassB;

index.js

const ClassA = require('./ClassA');

ClassA.test();

When I run node index.js, I expect to see ClassA.test function in console, but it outputs ClassA.test undefined instead. Is it some undocumented thing, or it's a bug?

Ronin · Accepted Answer · 2019-12-23 06:29:18Z

1

You have a circular dependency. The solution is to have a third class that will require both and use whatever code you have from ClassA in ClassB and the other way around.

const ClassA = require('./ClassA');
const ClassB = require('./ClassB');

class ClassC {
    static test() {
        ClassA.test();
        ClassB.test();
    }
}

When I run node index.js, I expect to see ClassA.test function in console, but it outputs ClassA.test undefined instead. Is it some undocumented thing, or it's a bug?

No, it's not a bug, and it's not undocumented.

You can read more here on how it's handled: https://nodejs.org/api/modules.html#modules_cycles

You can also do a lazy load, and will solve your issues for this particular case. But in most cases, circular references are a symptom of poorly written code.

class ClassB {
    static test() {
        const ClassA = require('./ClassA')
        console.log('ClassA.test', ClassA.test);
    }
}

module.exports = ClassB;

edited Dec 23, 2019 at 6:29

Ronin

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answered Dec 23, 2019 at 3:27

Marcos Casagrande

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4 Comments

Ronin Over a year ago

Yeah, using const ClassA = require('./ClassA') inside the test() method — that was exactly how I fixed it some time ago. Just was interested, why it happens.

Marcos Casagrande Over a year ago

Yeah, it's a bit confusing when you first encounter the error, but it makes sense though :). Glad to help.

Ronin Over a year ago

I just realized why I was asking if it's undocumented or it's a bug: NodeJS usually is very generous for showing warnings/errors, but it keeps silence when circular dependency is happening. It just provides a damaged object without saying anything which is a bit unexpected behavior.

Marcos Casagrande Over a year ago

I agree about that, a warning would definitely be something positive.

CertainPerformance · Accepted Answer · 2019-12-23 03:28:07Z

1

You have a circular dependency, so Node can't know for certain which module's top-level code should run first - one of the modules needs to have its top-level code run first so that something should be exported so that the other module can import it via require. But if both depend on the other, then the first one that runs will have its require call to the other resolve to undefined.

The best solution would be to refactor it to avoid the circular dependency.

answered Dec 23, 2019 at 3:28

CertainPerformance

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