CODEWITHSUNDEEP
javascriptnode.js
HansMu158
HansMu158

Reputation: 555

Not able to access another module's function in NodeJS

To break it down and simplify it a bit: Imagine I have three files in my project. One main.js, and two modules: moduleA.js and moduleB.js. main.js accesses moduleA.js which calls a function from moduleB.js. Now moduleB.js finds out that it needs an information that is only available in moduleA.js. Of course moduleB.js tries to access a function in moduleA.js which is technically able to give this information to moduleB.js but there is an error.

Here is the simplified code.

main.js

var a = require("./moduleA.js");

console.log(a.respond());

moduleA.js

var b = require("./moduleB.js");

module.exports = {
    respond: function(){
        return b.returnAnswerForModuleA();
    },
    getSomeInformationOnlyAvailableInA: function(){
        return "This is the information we need!";
    }
};

moduleB.js

var a = require("./moduleA.js");

module.exports = {
  returnAnswerForModuleA: function(){
      return a.getSomeInformationOnlyAvailableInA()
  }
};

Here is the error message:

/Users/Tim/Code/ChatBot/test/moduleB.js:5
      return a.getSomeInformationOnlyAvailableInA()
               ^

TypeError: a.getSomeInformationOnlyAvailableInA is not a function
    at Object.module.exports.returnAnswerForModuleA (/Users/Tim/Code/ChatBot/test/moduleB.js:5:16)
    at Object.module.exports.respond (/Users/Tim/Code/ChatBot/test/moduleA.js:5:18)
    at Object.<anonymous> (/Users/Tim/Code/ChatBot/test/main.js:3:15)
    at Module._compile (module.js:425:26)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:457:10)
    at startup (node.js:136:18)
    at node.js:972:3

Why cannot I access moduleA.js from moduleB.js?

To restructure my code is not really an option!

Thanks for help!

Upvotes: 1

Views: 198

Answers (1)

jfriend00
jfriend00

Reputation: 708176

This looks like an issue with circular dependencies in node.js where a circular dependency causes one module to be resolved as an empty object (thus causing the error you see). See these articles which explain:

Node.js and circular dependencies

Circular dependencies in Node.js

The first article offers two possible solutions: "delaying invocation of dependency until runtime" and "replace circular dependency with dependency injection".

I think you can work around the circular issue by changing module B to this:

var moduleA;

module.exports = {
  returnAnswerForModuleA: function(){
      if (!moduleA) {
          moduleA = require("./moduleA.js");
      }
      return moduleA.getSomeInformationOnlyAvailableInA()
  }
};

This delays the loading of module A in module B until runtime by which time, it is already successfully loaded and in the module cache, thus avoiding the circular dependency.

Upvotes: 2

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