Is there a way to simulate the parameters of a method callback from an external dependency in Nodejs

Question

Is there a way to simulate the parameters of a method callback from an external dependency in Nodejs

Imagine a scenario where I have the following structure:

lib/modules/module1.js

var m2 = require('module2');

module.exports = function(){
    return {
        // ...
        get: function(cb){
            m2.someMethod(params, function(error, data){
                if(error){
                    cb(error)
                }

                cb(null, data)
            })
        },
        // ...
    }
}

Now consider that there is another directory called tests/testModule1.js. Within this file, I create an instantiation of module1 in order to conduct certain tests.

I am interested in mocking the objects passed by the m2.someMethod to its callback function (not the cb function), specifically from the file testModule1.js.

I have researched about Sinon.js, but I haven't been able to find out if it's actually possible. Does anyone know if this can be achieved using Sinon.js?

Thank you.

javascript node.js unit-testing mocking

Answer 1

Answer №1

One possible solution is using a library like proxyquire, although I'm not particularly fond of modifying the built-in require.

Personally, my suggestion would be to refactor your code and implement dependency injection:

module.exports = function(service){
    return {
        // ...
        get: function(callback){
            service.someMethod(params, function(error, data){
                if(error){
                    callback(error);
                }

                callback(null, data);
            });
        },
        // ...
    }
}

Note that in this version, service is now passed as a parameter to the exported function. Then, in another part of your application (such as app.js or main.js), you can do the following:

app.js

var module1Creator = require('module1');
var module2 = require('module2');
var module1 = module1Creator(module2);

When it comes time for testing...

testModule1.js

var module1Creator = require('module1');
// inject the "fake" version containing test data, spies, etc
var mockedModule2 = require('mockedModule2');
var module1 = module1Creator(mockedModule2);

Answer 2

One possible solution is using a library like proxyquire, although I'm not particularly fond of modifying the built-in require.

Personally, my suggestion would be to refactor your code and implement dependency injection:

module.exports = function(service){
    return {
        // ...
        get: function(callback){
            service.someMethod(params, function(error, data){
                if(error){
                    callback(error);
                }

                callback(null, data);
            });
        },
        // ...
    }
}

Note that in this version, service is now passed as a parameter to the exported function. Then, in another part of your application (such as app.js or main.js), you can do the following:

app.js

var module1Creator = require('module1');
var module2 = require('module2');
var module1 = module1Creator(module2);

When it comes time for testing...

testModule1.js

var module1Creator = require('module1');
// inject the "fake" version containing test data, spies, etc
var mockedModule2 = require('mockedModule2');
var module1 = module1Creator(mockedModule2);

Answer 3

Answer №2

I usually agree with the idea of changing the design, and like @dvlsg suggested, DI would be my preference as well.

However, I'm currently working on a project that is already in progress and quite extensive. Making this change would require a significant amount of manpower, which may not be justified in this particular case.

Fortunately, I have discovered a solution. Whenever you use the require('someModule') function, the someModule is loaded and stored as a singleton in a global cache (although I admit I don't fully comprehend this mechanism yet). This means that no matter where you use require('someModule') from, you will receive the same cached version.

To illustrate, if in lib/modules/module1.js I utilize the require('module2') function, it loads and stores module2 in the cache. From there, I can also mock require('module2') in tests/testModule1.js. This modification will affect the outcome when calling get() from lib/modules/module1.js.

In order to achieve this, I made use of Sinon.js to create the mocks within the test files.

The aforementioned procedure has effectively resolved my problem without necessitating a complete redesign. Additionally, it allowed me to conduct the necessary tests. Consequently, I am sharing this response. However, please note that I won't mark it as the accepted answer because, as previously mentioned, I still have some uncertainty regarding this mechanism and changing required modules is generally discouraged.

I am interested to hear the thoughts of other developers on this topic. If the discussion leads to unanimous agreement, I will ultimately mark this approach as the accepted answer.

Answer 4

I usually agree with the idea of changing the design, and like @dvlsg suggested, DI would be my preference as well.

However, I'm currently working on a project that is already in progress and quite extensive. Making this change would require a significant amount of manpower, which may not be justified in this particular case.

Fortunately, I have discovered a solution. Whenever you use the require('someModule') function, the someModule is loaded and stored as a singleton in a global cache (although I admit I don't fully comprehend this mechanism yet). This means that no matter where you use require('someModule') from, you will receive the same cached version.

To illustrate, if in lib/modules/module1.js I utilize the require('module2') function, it loads and stores module2 in the cache. From there, I can also mock require('module2') in tests/testModule1.js. This modification will affect the outcome when calling get() from lib/modules/module1.js.

In order to achieve this, I made use of Sinon.js to create the mocks within the test files.

The aforementioned procedure has effectively resolved my problem without necessitating a complete redesign. Additionally, it allowed me to conduct the necessary tests. Consequently, I am sharing this response. However, please note that I won't mark it as the accepted answer because, as previously mentioned, I still have some uncertainty regarding this mechanism and changing required modules is generally discouraged.

I am interested to hear the thoughts of other developers on this topic. If the discussion leads to unanimous agreement, I will ultimately mark this approach as the accepted answer.

Is there a way to simulate the parameters of a method callback from an external dependency in Nodejs

Answer №1

app.js

testModule1.js

Answer №2

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