Question

I am struggling to write high quality tests around my node modules. The problem is the require module system. I want to be able to check that a certain required module has a method or its state has changed. There seem to be 2 relatively small libraries which can be used here: node-gently and mockery. However, due to their low ‘profile’ it makes me think that either people don’t test this, or there is another way of doing this that I am not aware of.

What is the best way to mock out and test a module that has been required?

Answer 1

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———– UPDATE —————

node-sandbox works on the same principals as stated below but is wrapped up in a nice module. I am finding it very nice to work with.

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————— detailed awnser —————

After much trial I have found the best way to test node modules in isolation while mocking things out is to use the method by Vojta Jina to run each module inside of a vm with a new context as explained here.

with this testing vm module:

var vm = require('vm');
var fs = require('fs');
var path = require('path');

/**
 * Helper for unit testing:
 * - load module with mocked dependencies
 * - allow accessing private state of the module
 *
 * @param {string} filePath Absolute path to module (file to load)
 * @param {Object=} mocks Hash of mocked dependencies
 */
exports.loadModule = function(filePath, mocks) {
  mocks = mocks || {};

  // this is necessary to allow relative path modules within loaded file
  // i.e. requiring ./some inside file /a/b.js needs to be resolved to /a/some
  var resolveModule = function(module) {
    if (module.charAt(0) !== '.') return module;
    return path.resolve(path.dirname(filePath), module);
  };

  var exports = {};
  var context = {
    require: function(name) {
      return mocks[name] || require(resolveModule(name));
    },
    console: console,
    exports: exports,
    module: {
      exports: exports
    }
  };

  vm.runInNewContext(fs.readFileSync(filePath), context);
  return context;
};

it is possible to test each module with its own context and easily stub out all external dependencys.

fsMock = mocks.createFs();
mockRequest = mocks.createRequest();
mockResponse = mocks.createResponse();

// load the module with mock fs instead of real fs
// publish all the private state as an object
module = loadModule('./web-server.js', {fs: fsMock});

I highly recommend this way for writing effective tests in isolation. Only acceptance tests should hit the entire stack. Unit and integration tests should test isolated parts of the system.