Dependency injection for node.js applications

npm install rewire
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Dependency injection for node.js applications.

rewire adds a special setter and getter to modules so you can modify their behaviour for better unit testing. You may

  • inject mocks for other modules or globals like process
  • leak private variables
  • override variables within the module.

rewire does not load the file and eval the contents to emulate node's require mechanism. In fact it uses node's own require to load the module. Thus your module behaves exactly the same in your test environment as under regular circumstances (except your modifications).

Good news to all caffeine-addicts: rewire works also with Coffee-Script. Note that in this case CoffeeScript needs to be listed in your devDependencies.

If you want to use rewire also on the client-side take a look at client-side bundlers

Build Status Dependency Status


npm install rewire


Imagine you want to test this module:

// lib/myModule.js

// With rewire you can change all these variables
var fs = require("fs"),
    http = require("http"),
    someOtherVar = "hi",
    myPrivateVar = 1;

function readSomethingFromFileSystem(cb) {
    // But no scoped variables
    var path = "/somewhere/on/the/disk";

    console.log("Reading from file system ...");
    fs.readFile(path, "utf8", cb);

exports.readSomethingFromFileSystem = readSomethingFromFileSystem;

Now within your test module:

// test/myModule.test.js

var rewire = require("rewire");

// rewire acts exactly like require.
var myModule = rewire("../lib/myModule.js");

// Just with one difference:
// Your module will now export a special setter and getter for private variables.
myModule.__set__("myPrivateVar", 123);
myModule.__get__("myPrivateVar"); // = 123

// This allows you to mock almost everything within the module e.g. the fs-module.
// Just pass the variable name as first parameter and your mock as second.
myModule.__set__("fs", {
    readFile: function (path, encoding, cb) {
        cb(null, "Success!");
myModule.readSomethingFromFileSystem(function (err, data) {
    console.log(data); // = Success!

// You can set different variables with one call.
    fs: fsMock,
    http: httpMock,
    someOtherVar: "hello"

// You may also override globals. These changes are only within the module, so
// you don't have to be concerned that other modules are influenced by your mock.
    console: {
        log: function () { /* be quiet */ }
    process: {
        argv: ["testArg1", "testArg2"]

// But be careful, if you do something like this you'll change your global
// console instance.
myModule.__set__("console.log", function () { /* be quiet */ });

// There is another difference to require:
// Every call of rewire() returns a new instance.
rewire("./myModule.js") === rewire("./myModule.js"); // = false


rewire(filename): rewiredModule

  • filename:
    Path to the module that shall be rewired. Use it exactly like require().

rewiredModule.__set__(name, value)

  • name:
    Name of the variable to set. The variable should be global or defined with var in the top-leve scope of the module.
  • value:
    The value to set.


  • env:
    Takes all keys as variable names and sets the values respectively.

rewiredModule.__get__(name): value

Returns the private variable.

Client-Side Bundlers


See rewire-webpack


If you're using browserify and want to use rewire with browserify please let me know.



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