CODEWITHSUNDEEP
unit-testingnode.jsmocking
JamesEggers
JamesEggers

Reputation: 12936

Mocking a dependency in Node

I'm in the process of learning Node.js and am wondering about how people mock dependencies in their modules when unit testing.

For example: I have a module that abstracts my MongoDB calls. A module that uses this module may start out something like this.

var myMongo = require("MyMongoModule");
// insert rest of the module here.

I want to ensure I test such a module in isolation while also ensuring that my tests don't insert records/documents into Mongo.

Is there a module/package that I can use that proxies require() so I can inject in my own mocks? How do other's typically address this issue?

Upvotes: 4

Views: 2182

Answers (4)

analog-nico
analog-nico

Reputation: 2780

Overwriting require to inject your mocks is a possible solution. However, I concur in Raynos' opinion:

I personally find the methodology of overwriting require on a file by file basis an "ugly hack" and prefer to go for proper DI. It is however optimum for mocking one or two modules on an existing code base without rewriting code for DI support.

To use proper dependency injection not only saves you an "ugly hack" but also allows you to apply additional use cases apart from injecting mocks. In production you may e.g. usually instantiate connections over http and in certain circumstances inject a different implementation to establish a connection over VPN.

If you want to look for a dependency injection container read this excellent article and check out Fire Up! which I implemented.

Upvotes: 0

Lars-Erik Roald
Lars-Erik Roald

Reputation: 145

You easily mock require by using "a": https://npmjs.org/package/a

e.g. need to mock require('./foo') in unit test:
var fakeFoo = {};
var expectRequire = require('a').expectRequire;
expectRequire('./foo).return(fakeFoo);

//in sut:
var foo = require('./foo); //returns fakeFoo

Upvotes: 0

JamesEggers
JamesEggers

Reputation: 12936

After reviewing Ryanos's suggestion as well as the Horaa package on npm, I discovered this thread on the Google Group that pointed me towards Sandboxed-Module.

Sandboxed-Module allows me to inject/override require() without me having to expose such dependencies for my unit tests.

I'm still up for other suggestions; however, Sandboxed-Module appears to fit my needs at the moment.

Upvotes: 0

Raynos
Raynos

Reputation: 169511

You can use a dependency injection library like nCore

To be honest, the hard part of this is actually mocking out the mongoDB API, which is complex and non trivial. I estimate it would take about a week to mock out most of the mongo API I use so I just test againts the a local mongodb database on my machine (which is always in a weird state)

Then with nCore specific syntax

// myModule.js
module.exports = {
  myMethod: function () { 
    this.mongo.doStuff(...)
  },
  expose: ["myMethod"]
};

// test-myModule.js
var module = require("myModule")

module.mongo = mongoMock
assert(module.myMethod() === ...)

Upvotes: 1

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