This page describes how to isolate your system under test, by targetting the link seams; replacing your dependencies with stubs you control.

If you want a better understand of the example and get a good description of what seams are, we recommend that you read the seams (all 3 web pages) excerpt from the classic Working Effectively with Legacy Code, though it is not strictly necessary.

This guide targets the CommonJS module system, made popular by NodeJS. There are other module systems, but until recent years this was the de-facto module system and even when the actual EcmaScript Module standard arrived in 2015, transpilers and bundlers can still output code as CJS modules. For instance, Typescript outputs CJS modules per default as of 2023, so it is still relevant, as your import foo from './foo' might still end up being transpiled into const foo = require('./foo') in the end.

Hooking into require

For us to replace the underlying calls done by require we need a tool to hook into the process. There are many tools that can do this: rewire, proxyquire, Quibble, etc. This example will be using proxyquire to construct our seams (i.e. replace the modules), but the actual mechanics will be very similar for the other tools.

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");