This page describes how to isolate your system under test, by stubbing out dependencies with link seams.

This is the CommonJS version, so we will be using proxyquire to construct our seams.

To better understand the example and get a good description of what seams are, we recommend that you read the seams (all 3 web pages) excerpt from Working Effectively with Legacy Code before proceeding.

Read it?

Great! Let’s continue.


The folder structure in our example looks like this

├── lib
│   └── does-file-exist.js
└── test
    └── does-file-exist.test.js

Source and runnable demo of the example code.

Source file: lib/does-file-exist.js

This is the source file of the module doesFileExist, it only has one dependency: fs.

var fs = require("fs");

function doesFileExist(path) {
  return fs.existsSync(path);

module.exports = doesFileExist;

Test file: test/does-file-exist.test.js

In order to isolate our doesFileExist module for testing, we will stub out fs and provide a fake implementation of fs.existsSync, where we have complete control of the behaviour.

var proxyquire = require("proxyquire");
var sinon = require("sinon");
var assert = require("referee").assert;

var doesFileExist; // the module to test
var existsSyncStub; // the fake method on the dependency

describe("example", function () {
  beforeEach(function () {
    existsSyncStub = sinon.stub(); // create a stub for every test

    // import the module to test, using a fake dependency
    doesFileExist = proxyquire("../lib/does-file-exist", {
      fs: {
        existsSync: existsSyncStub,

  describe("when a path exists", function () {
    beforeEach(function () {
      existsSyncStub.returns(true); // set the return value that we want

    it("should return `true`", function () {
      var actual = doesFileExist("9d7af804-4719-4578-ba1d-5dd8a4dae89f");